diff --git a/lib/www/server/lib/comparisons/geometric-differences.js b/lib/www/server/lib/comparisons/geometric-differences.js
new file mode 100644
index 0000000..bed21d1
--- /dev/null
+++ b/lib/www/server/lib/comparisons/geometric-differences.js
@@ -0,0 +1,563 @@
+const d3a = require('d3-array');
+const { DougalBinaryBundle } = require('@dougal/binary');
+const { pool, setSurvey } = require('../db/connection');
+const db = require('../db');
+const { bundle } = require('../binary/bundle');
+const setops = require('../utils/setops');
+const { ijRMS, combinations, computeSample } = require('./utils');
+const { computePCA } = require('./pca');
+const { ERROR, INFO, DEBUG } = require('DOUGAL_ROOT/debug')(__filename);
+
+
+async function fetchErrors (pid) {
+
+	const client = await setSurvey(pid);
+
+	try {
+		const text = `
+			SELECT
+				fs.line, fs.point,
+				ij_error(fs.line::double precision, fs.point::double precision, fs.geometry)::json AS errorfinal
+			FROM
+				final_shots fs
+			ORDER BY fs.line, fs.point;
+		`;
+
+		const res = await client.query(text);
+
+		return res.rows.map( row =>
+			[row.line, row.point, row.errorfinal.coordinates[0], row.errorfinal.coordinates[1]]
+		);
+	} catch (err) {
+		console.error(err);
+	} finally {
+		client.release();
+	}
+}
+
+
+async function groups () {
+	const projects = await db.project.get();
+	const groupNames = [
+		...projects
+			.reduce( (acc, cur) => acc.add(...cur.groups), new Set() )
+	].filter( i => !!i );
+
+	return Object.fromEntries(groupNames.map( g => [g, projects.filter( p => p.groups.includes(g) )] ));
+}
+
+/*
+async function compare (baselineProjectID, monitorProjectID) {
+	console.log("Getting baseline", baselineProjectID);
+	const baselineData = await db.sequence.get(baselineProjectID);
+	console.log("Getting monitor", monitorProjectID);
+	const monitorData = await db.sequence.get(monitorProjectID);
+	console.log("Comparing");
+
+	const comparison = comparisonGeometricDifferences(baselineData, monitorData);
+	return comparison;
+}
+*/
+
+function geometric_differences (baseline, monitor) {
+
+	if (!baseline || !baseline.length) {
+		throw new Error("No baseline data");
+	}
+
+	if (!monitor || !monitor.length) {
+		throw new Error("No monitor data");
+	}
+
+	const comparison = []; // An array of { line, point, εi, εj }; line + point may be repeated
+
+	for (const bp of baseline) {
+		const monitor_points = monitor.filter( mp => mp[0] === bp[0] && mp[1] === bp[1] );
+
+		if (!monitor_points.length) {
+			// console.log(`No match for L${bp[0]} P${bp[1]}`);
+			continue;
+		}
+
+		for (const mp of monitor_points) {
+			const εi = mp[2] - bp[2], εj = mp[3] - bp[3];
+			comparison.push([bp[0], bp[1], εi, εj]);
+		}
+
+	}
+
+	return comparison;
+}
+
+async function compare (baselineProjectID, monitorProjectID, infoObj) {
+	console.log("Getting baseline", baselineProjectID);
+	const baselineData = await fetchErrors(baselineProjectID);
+	console.log("Getting monitor", monitorProjectID);
+	const monitorData = await fetchErrors(monitorProjectID);
+	console.log("Comparing");
+
+	const comparison = geometric_differences(baselineData, monitorData);
+
+	if (infoObj instanceof Object) {
+		const baselineIJ = baselineData.map(i => i.slice(0,2));
+		const monitorIJ = monitorData.map(i => i.slice(0,2));
+
+		infoObj.compared = comparison.length;
+		infoObj.baselineLength = baselineData.length;
+		infoObj.monitorLength = monitorData.length;
+		infoObj.baselineUniqueLength = setops.unique(baselineIJ).length;
+		infoObj.monitorUniqueLength = setops.unique(monitorIJ).length;
+		infoObj.common = setops.intersection(baselineIJ, monitorIJ).length;
+	}
+
+	return comparison;
+}
+
+
+async function save (baselineProjectID, monitorProjectID, bundle, meta) {
+	const info = {};
+	if (!bundle) {
+		const comparison = await compare(baselineProjectID, monitorProjectID, info);
+		if (comparison.length) {
+			bundle = asBundle(comparison);
+		} else {
+			console.warn(`No matching points between ${baselineProjectID} and ${monitorProjectID}`);
+			return;
+		}
+	} else if (!(bundle instanceof DougalBinaryBundle)) {
+		throw new Error("Illegal data: `bundle` must of null or of type DougalBinaryBundle");
+	}
+
+	if (!bundle.byteLength) {
+		console.warn(`Empty comparison results between ${baselineProjectID} and ${monitorProjectID}. Refusing to store`);
+		return;
+	}
+
+	meta = {tstamp: (new Date()), ...info, ...stats(bundle), ...meta};
+
+	console.log("Storing in database");
+	const client = await pool.connect();
+
+	try {
+		const text = `
+			INSERT INTO comparisons.comparisons
+				(type, baseline_pid, monitor_pid, data, meta)
+			VALUES ('geometric_difference', $1, $2, $3, $4)
+			ON CONFLICT (type, baseline_pid, monitor_pid)
+			DO UPDATE SET
+				data = EXCLUDED.data,
+				meta = EXCLUDED.meta;
+		`;
+
+		const values = [ baselineProjectID, monitorProjectID, Buffer.from(bundle), meta ];
+		const res = await client.query(text, values);
+		return res.rowCount;
+	} catch (err) {
+		console.error(err);
+	} finally {
+		client.release();
+	}
+}
+
+async function saveSample (baselineProjectID, monitorProjectID, opts = {}) {
+	let sample = opts.sample;
+	let populationStats = opts.populationStats;
+	let sampleStats = opts.sampleStats;
+
+	if (!sample?.length) {
+		const sampleSize = opts.sampleSize ?? 2000;
+		const record = await get(baselineProjectID, monitorProjectID);
+		let data;
+
+		if (record) {
+			data = record.data;
+		} else {
+			console.log("Full data not found in database");
+			data = asBundle(await compare(baselineProjectID, monitorProjectID));
+		}
+
+		sample = computeSample(data, opts);
+
+		if (!populationStats) {
+			populationStats = stats(data);
+		}
+	}
+
+	const bundle = asBundle(sample, {type: 0x0c});
+
+	if (!sampleStats) {
+		sampleStats = stats(bundle);
+	}
+
+	meta = {tstamp: (new Date()), populationStats, sampleStats, ...(opts.meta??{})};
+
+	const client = await pool.connect();
+
+	try {
+		const text = `
+			INSERT INTO comparisons.comparisons
+				(type, baseline_pid, monitor_pid, data, meta)
+			VALUES ('geometric_difference_sample', $1, $2, $3, $4)
+			ON CONFLICT (type, baseline_pid, monitor_pid)
+			DO UPDATE SET
+				data = EXCLUDED.data,
+				meta = EXCLUDED.meta;
+		`;
+
+		const values = [ baselineProjectID, monitorProjectID, Buffer.from(bundle), meta ];
+		const res = await client.query(text, values);
+		return res.rowCount;
+	} catch (err) {
+		console.error(err);
+	} finally {
+		client.release();
+	}
+}
+
+async function get (baselineProjectID, monitorProjectID, type = 'geometric_difference') {
+
+	const client = await pool.connect();
+
+	try {
+
+		const text = `
+			SELECT data, meta
+			FROM comparisons.comparisons
+			WHERE type = $3 AND baseline_pid = $1 AND monitor_pid = $2;
+		`;
+
+		const values = [ baselineProjectID, monitorProjectID, type ];
+		const res = await client.query(text, values);
+		if (!res.rows.length) {
+			console.log("Comparison not found in database");
+			return;
+		}
+
+		const { data, meta } = res.rows[0];
+		return {
+			data: DougalBinaryBundle.clone(data),
+			meta
+		};
+	} catch (err) {
+		console.error(err);
+	} finally {
+		client.release();
+	}
+}
+
+
+async function getSample (baselineProjectID, monitorProjectID) {
+	return await get(baselineProjectID, monitorProjectID, 'geometric_difference_sample');
+}
+
+
+async function remove (baselineProjectID, monitorProjectID) {
+	const client = await pool.connect();
+
+	try {
+		const text = `
+			DELETE
+			FROM comparisons.comparisons
+			WHERE
+				(type = 'geometric_difference' OR type = 'geometric_difference_sample')
+				AND baseline_pid = $1
+				AND monitor_pid = $2;
+		`;
+
+		const values = [ baselineProjectID, monitorProjectID ];
+
+		const res = await client.query(text, values);
+		return res.rowCount;
+	} catch (err) {
+		console.error(err);
+	} finally {
+		client.release();
+	}
+}
+
+function stats (comparison) {
+	let i, j, δi, δj;
+
+	if (comparison instanceof DougalBinaryBundle) {
+		console.log("Computing stats");
+		const udv = comparison.chunks()[0]?.udv;
+
+		if (!udv) {
+			console.error("Could not determine udv from first chunk");
+			console.log(comparison.chunks());
+			return;
+		}
+
+		let records;
+
+		if (udv == 0xa) {
+			records = comparison.records;
+
+			// Transpose the records
+			[ i, j, δi, δj ] = Array.from({ length: 4 }, (_, i) => records.map(row => row[i]));
+		} else if (udv == 0xc) {
+			records = comparison.records;
+			let _;
+			[ _, _, i, j, δi, δj ] = Array.from({ length: 6 }, (_, i) => records.map(row => row[i]));
+		} else {
+			throw new Error(`Unrecognised DougalBinaryBundle User Defined Value: ${udv}`);
+		}
+
+		return {
+			length: records.length,
+			μ: [ d3a.mean(δi), d3a.mean(δj) ],
+			σ: [ d3a.deviation(δi), d3a.deviation(δj) ],
+			//rms: ijRMS(δi, δj),
+			...computePCA(records)
+		}
+	} else if (Array.isArray(comparison)) {
+		if (Array.isArray(comparison[0])) {
+			return stats(asBundle(comparison, {type: 0xc}));
+		} else {
+			// Assume object
+			return stats(asBundle(comparison));
+		}
+	}
+}
+
+
+/** Compare two projects' errorfinal quantities.
+ *
+ * Assumes that the preplots are the same.
+ * It is not a terribly efficient way of doing it, but considering
+ * that this is, by and large only going to be done once every few
+ * hours for an active prospect, and never for inactive ones, I
+ * think and hope we can live with that.
+ *
+ * `baseline` and `monitor` are the result of calling
+ * db.sequence.get(projectId) on each of the respective
+ * projects.
+ */
+/*
+function comparisonGeometricDifferences (baseline, monitor) {
+
+	if (!baseline || !baseline.length) {
+		throw new Error("No baseline data");
+	}
+
+	if (!monitor || !monitor.length) {
+		throw new Error("No monitor data");
+	}
+
+	const comparison = []; // An array of { line, point, εi, εj, δts }; line + point may be repeated
+
+	for (const bp of baseline) {
+		if (!bp.errorfinal) {
+			console.log(`No final data for baseline point L${bp.line} S${bp.sequence} P${bp.point}`);
+			continue;
+		}
+
+		const monitor_points = monitor.filter( mp => mp.line === bp.line && mp.point === bp.point );
+		for (const mp of monitor_points) {
+			if (!mp.errorfinal) {
+				console.log(`No final data for monitor point L${mp.line} S${mp.sequence} P${mp.point}`);
+				continue;
+			}
+
+			const line = bp.line;
+			const point = bp.point;
+			const baseSeq = bp.sequence;
+			const monSeq = mp.sequence;
+			const baseTStamp = bp.tstamp;
+			const monTStamp = mp.tstamp;
+			const δi = bp.errorfinal.coordinates[0] - mp.errorfinal.coordinates[0];
+			const δj = bp.errorfinal.coordinates[1] - mp.errorfinal.coordinates[1];
+
+			const obj = {line, point, baseSeq, monSeq, baseTStamp, monTStamp, δi, δj};
+			comparison.push(obj);
+			// console.log(obj);
+		}
+	}
+
+	return comparison.sort(sortFn);
+}
+
+function sortComparison (comparison) {
+	return comparison.sort( (a, b) => {
+		if (a.line == b.line) {
+			if (a.point == b.point) {
+				return a.baseTStamp - b.baseTStamp;
+			} else {
+				return a.point - b.point;
+			}
+		} else {
+			return a.line - b.line;
+		}
+	})
+}
+*/
+
+function sortFn (a, b) {
+	if (a.line == b.line) {
+		if (a.point == b.point) {
+			return a.baseTStamp - b.baseTStamp;
+		} else {
+			return a.point - b.point;
+		}
+	} else {
+		return a.line - b.line;
+	}
+}
+
+function asBundle (comparison, opts = {type: 0x0a}) {
+	return DougalBinaryBundle.clone(bundle(comparison, opts));
+}
+
+function fromBundle (bundle) {
+	if (!(bundle instanceof DougalBinaryBundle)) {
+		bundle = DougalBinaryBundle.clone(bundle);
+	}
+
+	const json = [];
+	for (const record of bundle) {
+		record.shift();
+		json.push(record);
+	}
+	return json;
+}
+
+async function saveGroup (group, opts = {}) {
+	if (group == null) {
+		// Save everything
+		const g = await groups();
+		for (const group of Object.values(g)) {
+			await saveGroup(group)
+		}
+	} if (typeof group === "string") {
+		// This is a group name
+		const g = await groups();
+		group = groups[g];
+	}
+
+	if (Array.isArray(group)) {
+		const pids = group.map( i => i.pid ).sort();
+
+		for (const [ baselineProjectID, monitorProjectID ] of combinations(pids, 2)) {
+			try {
+				const isSaved = await save(baselineProjectID, monitorProjectID);
+				if (isSaved) {
+					await saveSample(baselineProjectID, monitorProjectID, opts.sampleOpts);
+				} else {
+					await remove(baselineProjectID, monitorProjectID);
+				}
+				DEBUG("Saved comparison between %s and %s", baselineProjectID, monitorProjectID);
+			} catch (err) {
+				console.error(err);
+				ERROR("Error saving comparison between %s and %s", baselineProjectID, monitorProjectID);
+			}
+		}
+	}
+}
+
+/*
+async function getGroup (groupName, opts = {}) {
+
+	const group = (await groups())?.[groupName]?.map( i => i.pid)?.sort();
+
+	if (!group?.length) return;
+
+	const client = await pool.connect();
+
+	try {
+
+		const text = `
+			-- SQL query goes here
+		`;
+
+		const values = combinations(group, 2);
+		const res = await client.query(text, values);
+		if (!res.rows.length) {
+			console.log("Comparison not found in database");
+			return;
+		}
+
+		if (opts.returnData) {
+			return res.rows.map( row => ({
+				data: DougalBinaryBundle.clone(row.data),
+				meta: row.meta
+			});
+		} else {
+			return res.rows.map( row => row.meta );
+		}
+	} catch (err) {
+		console.error(err);
+	} finally {
+		client.release();
+	}
+}
+*/
+
+
+async function getGroup (groupName, opts = {}) {
+
+	const group = (await groups())?.[groupName]?.map( i => i.pid)?.sort();
+
+	if (!group?.length) return;
+
+	const client = await pool.connect();
+
+	try {
+
+		const pairs = combinations(group, 2);
+		const flatValues = pairs.flat();
+		const placeholders = [];
+		for (let i = 0; i < pairs.length; i++) {
+			placeholders.push(`($${i * 2 + 1}, $${i * 2 + 2})`);
+		}
+		const inClause = placeholders.join(',');
+		const selectFields = opts.returnData ? 'data, meta' : 'meta';
+
+		const text = `
+			SELECT baseline_pid, monitor_pid, ${selectFields}
+			FROM comparisons.comparisons
+			WHERE type = 'geometric_difference'
+			AND (baseline_pid, monitor_pid) IN (VALUES ${inClause})
+			ORDER BY baseline_pid, monitor_pid
+		`;
+
+		console.log(text);
+		console.log(flatValues);
+
+		const res = await client.query(text, flatValues);
+		if (!res.rows.length) {
+			console.log("Comparison not found in database");
+			return;
+		}
+
+		if (opts.returnData) {
+			return res.rows.map( row => ({
+				...row,
+				data: DougalBinaryBundle.clone(row.data),
+			}));
+		} else {
+			return res.rows;
+		}
+	} catch (err) {
+		console.error(err);
+	} finally {
+		client.release();
+	}
+}
+
+module.exports = {
+	groups,
+	fetchErrors,
+	compare,
+	computeSample,
+	get,
+	save,
+	getSample,
+	saveSample,
+	saveGroup,
+	getGroup,
+	remove,
+	stats,
+	// comparisonGeometricDifferences,
+	asBundle,
+	fromBundle
+};
diff --git a/lib/www/server/lib/comparisons/index.js b/lib/www/server/lib/comparisons/index.js
new file mode 100644
index 0000000..53e05d4
--- /dev/null
+++ b/lib/www/server/lib/comparisons/index.js
@@ -0,0 +1,4 @@
+
+module.exports = {
+	...require('./geometric-differences')
+}
diff --git a/lib/www/server/lib/comparisons/pca.js b/lib/www/server/lib/comparisons/pca.js
new file mode 100644
index 0000000..f4303b0
--- /dev/null
+++ b/lib/www/server/lib/comparisons/pca.js
@@ -0,0 +1,80 @@
+const math = require('mathjs');
+
+/**
+ * Compute PCA (eigenvectors and eigenvalues) for deviation data to assess geometric repeatability.
+ * @param {Array<Array<number>>} deviations - Array of [point, line, i_deviation, j_deviation]
+ * @returns {Object} - { eigenvalues, eigenvectors, rms, anisotropy, primaryDirection }
+ */
+function computePCA(deviations) {
+	// Extract i_deviation and j_deviation
+	const deviationMatrix = deviations.map(row => [row[2], row[3]]);
+
+	// Convert to mathjs matrix
+	const D = math.matrix(deviationMatrix);
+
+	// Compute mean for centering (1 x 2 matrix)
+	const mean = math.mean(D, 0);
+
+	// Explicitly repeat mean to match D's shape (n x 2)
+	const n = deviationMatrix.length;
+	const meanRepeated = math.repmat(mean, n, 1);
+
+	// Center the data
+	const centered = math.subtract(D, meanRepeated);
+
+	// Compute covariance matrix: (1/(n-1)) * (D_centered^T * D_centered)
+	const covMatrix = math.multiply(
+		math.multiply(1 / (n - 1), math.transpose(centered)),
+		centered
+	);
+
+	// Perform eigen decomposition
+	const result = math.eigs(covMatrix);
+	let eigenvalues = result.values;
+	const evObjs = result.eigenvectors;
+
+	// Convert eigenvalues to array if it's a matrix
+	eigenvalues = Array.isArray(eigenvalues) ? eigenvalues : eigenvalues.toArray();
+
+	// Create pairs and convert vector to array if necessary
+	const pairs = eigenvalues.map((val, i) => {
+		let vec = evObjs[i].vector;
+		if (vec.toArray) vec = vec.toArray();
+		return { val, vec };
+	});
+
+	// Sort by descending eigenvalues
+	pairs.sort((a, b) => b.val - a.val);
+
+	// Sorted eigenvalues
+	const sortedEigenvalues = pairs.map(p => p.val);
+
+	// Build eigenvector matrix: rows as components, columns as eigenvectors
+	const dimension = pairs[0].vec.length; // e.g., 2
+	const evecRows = [];
+	for (let comp = 0; comp < dimension; comp++) {
+		evecRows.push(pairs.map(p => p.vec[comp]));
+	}
+	const sortedEigenvectors = math.matrix(evecRows);
+
+	// Compute RMS errors along principal axes
+	const rms = sortedEigenvalues.map(val => Math.sqrt(Math.max(val, 0)));
+
+	// Compute anisotropy (ratio of major to minor axis variance)
+	const anisotropy = sortedEigenvalues[0] / (sortedEigenvalues[1] || 1); // Avoid division by zero
+
+	// Primary direction (angle in degrees of major eigenvector)
+	const primaryVector = sortedEigenvectors.subset(math.index([0, 1], 0)).toArray();
+	const primaryDirection = Math.atan2(primaryVector[1], primaryVector[0]) * 180 / Math.PI;
+
+	return {
+		eigenvalues: sortedEigenvalues,
+		eigenvectors: sortedEigenvectors.toArray(),
+		rms: rms, // RMS errors along major/minor axes
+		anisotropy: anisotropy, // Ratio of variances
+		primaryDirection: primaryDirection // Angle of major axis (degrees)
+	};
+}
+
+
+module.exports = { computePCA };
diff --git a/lib/www/server/lib/comparisons/utils.js b/lib/www/server/lib/comparisons/utils.js
new file mode 100644
index 0000000..64f6276
--- /dev/null
+++ b/lib/www/server/lib/comparisons/utils.js
@@ -0,0 +1,310 @@
+const d3 = require('d3-array');
+
+// Function to calculate the root mean square (RMS) of position deviations
+// This computes the RMS of the Euclidean distances: sqrt( (1/n) * sum(δi² + δj²) )
+// Assumes deviations are already centered (mean deviation ~0); if normalization by std dev or range is needed, adjust accordingly
+function ijRMS(δi, δj) {
+	if (!δi.length || !δj.length) return 0;
+
+	if (δi.length != δj.length) {
+		console.warn(`δi and δj have different lengths!`);
+	}
+
+	let sumSquares = 0;
+	const n = Math.min(δi.length, δj.length);
+
+	for (let i=0; i < n; i++) {
+		sumSquares += (δi[i] * δi[i]) + (δj[i] * δj[i]);
+	}
+
+	const meanSquare = sumSquares / n;
+	const rms = Math.sqrt(meanSquare);
+
+	return rms;
+}
+
+
+/**
+ * Performs stratified sampling on an array of [line, point, δi, δj] data points.
+ * Groups by line and samples proportionally to preserve shape and spread.
+ *
+ * @param {Array<Array<number>>} data - Input data: [[line, point, δi, δj], ...]
+ * @param {number} sampleSize - Target number of samples (e.g., 2000)
+ * @returns {Array<Array<number>>} Sampled data in same format
+ */
+function old_stratifiedSample(data, sampleSize) {
+	if (!Array.isArray(data) || data.length === 0) return [];
+	if (!Number.isInteger(sampleSize) || sampleSize <= 0) {
+		throw new Error('sampleSize must be a positive integer');
+	}
+
+	// Group data by line (first element)
+	const grouped = d3.group(data, d => d[0]);
+	const totalSize = data.length;
+	const sampled = [];
+
+	// Ensure sampleSize doesn't exceed data size
+	const effectiveSampleSize = Math.min(sampleSize, totalSize);
+
+	// Iterate over each line group
+	for (const [line, group] of grouped) {
+		// Calculate proportional sample size for this group
+		const groupSize = group.length;
+		const groupSampleSize = Math.max(1, Math.round((groupSize / totalSize) * effectiveSampleSize));
+
+		// Shuffle group and take first N elements
+		const shuffled = d3.shuffle([...group]);
+		sampled.push(...shuffled.slice(0, groupSampleSize));
+	}
+
+	// If sampled size is slightly off due to rounding, adjust
+	if (sampled.length > effectiveSampleSize) {
+		return d3.shuffle(sampled).slice(0, effectiveSampleSize);
+	} else if (sampled.length < effectiveSampleSize) {
+		// Pad with random samples from entire dataset if needed
+		const remaining = effectiveSampleSize - sampled.length;
+		const additional = d3.shuffle(data.filter(d => !sampled.includes(d))).slice(0, remaining);
+		sampled.push(...additional);
+	}
+
+	return sampled;
+}
+
+/**
+ * Performs stratified sampling on an array of [line, point, δi, δj] data points.
+ * Stratifies by line and δi quantiles to preserve shape and spread, with outlier control.
+ *
+ * @param {Array<Array<number>>} data - Input data: [[line, point, δi, δj], ...]
+ * @param {number} sampleSize - Target number of samples (e.g., 2000)
+ * @param {number} [binsPerLine=10] - Number of δi quantile bins per line
+ * @returns {Array<Array<number>>} Sampled data in same format
+ */
+function stratifiedSample(data, sampleSize, binsPerLine = 10) {
+	if (!Array.isArray(data) || data.length === 0) return [];
+	if (!Number.isInteger(sampleSize) || sampleSize <= 0) {
+		throw new Error('sampleSize must be a positive integer');
+	}
+	if (!Number.isInteger(binsPerLine) || binsPerLine <= 0) {
+		throw new Error('binsPerLine must be a positive integer');
+	}
+
+	const totalSize = data.length;
+	const effectiveSampleSize = Math.min(sampleSize, totalSize);
+	const sampled = [];
+
+	// Group by line
+	const groupedByLine = d3.group(data, d => d[0]);
+
+	// Compute population stats for validation
+	const populationStats = computeStats(data);
+
+	// Iterate over each line
+	for (const [line, group] of groupedByLine) {
+		const groupSize = group.length;
+		const lineSampleSize = Math.max(1, Math.round((groupSize / totalSize) * effectiveSampleSize));
+
+		// Create quantile-based bins for δi
+		const δiValues = group.map(d => d[2]).sort(d3.ascending);
+		const quantiles = d3.range(0, binsPerLine + 1).map(i => d3.quantile(δiValues, i / binsPerLine));
+		const binnedData = group.map(d => {
+			const δi = d[2];
+			let binIndex = 0;
+			for (let i = 0; i < binsPerLine; i++) {
+				if (δi >= quantiles[i] && δi < quantiles[i + 1]) {
+					binIndex = i;
+					break;
+				}
+			}
+			return { data: d, bin: binIndex };
+		});
+		const groupedByBin = d3.group(binnedData, d => d.bin);
+
+		// Allocate samples across bins, inversely weighted by density to control outliers
+		const binSampleSizes = new Map();
+		let remainingLineSamples = lineSampleSize;
+		const binCounts = Array(binsPerLine).fill(0);
+		for (const [bin, binGroup] of groupedByBin) {
+			binCounts[bin] = binGroup.length;
+		}
+		const maxBinCount = d3.max(binCounts);
+		for (const [bin, binGroup] of groupedByBin) {
+			const binSize = binGroup.length;
+			// Inverse weighting: smaller bins (outliers) get fewer samples
+			const weight = binSize > 0 ? Math.max(0.1, 1 - (binSize / maxBinCount) * 0.5) : 1;
+			const binSampleSize = Math.max(1, Math.round(lineSampleSize * (binSize / groupSize) * weight));
+			binSampleSizes.set(bin, Math.min(binSampleSize, binSize));
+			remainingLineSamples -= binSampleSizes.get(bin);
+		}
+
+		// Distribute remaining samples
+		if (remainingLineSamples > 0) {
+			const nonEmptyBins = Array.from(groupedByBin.keys());
+			for (let i = 0; i < remainingLineSamples && nonEmptyBins.length > 0; i++) {
+				const bin = nonEmptyBins[i % nonEmptyBins.length];
+				binSampleSizes.set(bin, binSampleSizes.get(bin) + 1);
+			}
+		}
+
+		// Sample from each bin
+		for (const [bin, binGroup] of groupedByBin) {
+			const samples = d3.shuffle([...binGroup]).slice(0, binSampleSizes.get(bin)).map(s => s.data);
+			sampled.push(...samples);
+		}
+	}
+
+	// Adjust sample size
+	let finalSample = sampled;
+	if (sampled.length > effectiveSampleSize) {
+		finalSample = d3.shuffle(sampled).slice(0, effectiveSampleSize);
+	} else if (sampled.length < effectiveSampleSize) {
+		const remaining = effectiveSampleSize - sampled.length;
+		const additional = d3.shuffle(data.filter(d => !sampled.includes(d))).slice(0, remaining);
+		finalSample = [...sampled, ...additional];
+	}
+
+	// Validate and adjust if stats are off
+	const sampleStats = computeStats(finalSample);
+	const statTolerance = { μ: 0.1, σ: 0.2 }; // Allowable relative deviation
+	const needsAdjustment =
+		Math.abs(sampleStats.μ[0] - populationStats.μ[0]) / populationStats.μ[0] > statTolerance.μ ||
+		Math.abs(sampleStats.μ[1] - populationStats.μ[1]) / populationStats.μ[1] > statTolerance.μ ||
+		Math.abs(sampleStats.σ[0] - populationStats.σ[0]) / populationStats.σ[0] > statTolerance.σ ||
+		Math.abs(sampleStats.σ[1] - populationStats.σ[1]) / populationStats.σ[1] > statTolerance.σ;
+
+	if (needsAdjustment) {
+		// Add points from underrepresented regions
+		const δiSample = finalSample.map(d => d[2]);
+		const δiPopulation = data.map(d => d[2]);
+		const quantiles = d3.range(0, binsPerLine + 1).map(i => d3.quantile(δiPopulation, i / binsPerLine));
+		const sampleBins = d3.histogram().domain(d3.extent(δiPopulation)).thresholds(quantiles)(δiSample);
+		const populationBins = d3.histogram().domain(d3.extent(δiPopulation)).thresholds(quantiles)(δiPopulation);
+		const underSampledBins = sampleBins
+			.map((b, i) => ({ bin: i, diff: populationBins[i].length / totalSize - b.length / finalSample.length }))
+			.filter(b => b.diff > 0.1); // Significant under-sampling
+
+		if (underSampledBins.length > 0) {
+			const additionalSamples = [];
+			for (const { bin } of underSampledBins) {
+				const binData = data.filter(d => d[2] >= quantiles[bin] && d[2] < quantiles[bin + 1] && !finalSample.includes(d));
+				const needed = Math.round((underSampledBins[0].diff * effectiveSampleSize) / 2);
+				additionalSamples.push(...d3.shuffle(binData).slice(0, needed));
+			}
+			finalSample = d3.shuffle([...finalSample, ...additionalSamples]).slice(0, effectiveSampleSize);
+		}
+	}
+
+	return finalSample;
+}
+
+function decimate (data, decimationCount = 20) {
+	return data.filter( (row, index) => (index % decimationCount) == 0 );
+}
+
+function computeSample (data, opts = {}) {
+	const DEFAULT_SAMPLE_SIZE = 2000;
+	let sample;
+
+	if (opts.decimate === true) {
+		if (opts.sampleSize > 0) {
+			sample = decimate(data.records, Math.floor(data.records.length / opts.sampleSize));
+		} else {
+			sample = decimate(data.records, Math.floor(data.records.length / DEFAULT_SAMPLE_SIZE));
+		}
+	} else if (opts.decimate > 0) {
+		sample = decimate(data.records, opts.decimate);
+	} else if (opts.sampleSize) {
+		sample = stratifiedSample(data.records, opt.sampleSize);
+	} else {
+		sample = stratifiedSample(data.records, DEFAULT_SAMPLE_SIZE);
+	}
+
+	return sample;
+}
+
+// Optional: Utility to compute stats for validation
+function computeStats(data) {
+	const δi = data.map(d => d[2]);
+	const δj = data.map(d => d[3]);
+	const rms = Math.sqrt(d3.mean(data, d => d[2] ** 2 + d[3] ** 2));
+	return {
+		l: data.length,
+		μ: [d3.mean(δi), d3.mean(δj)],
+		σ: [d3.deviation(δi), d3.deviation(δj)],
+		rms
+	};
+}
+
+function centre (data) {
+	const stats = computeStats(data);
+
+	return data.map( row => [row[0], row[1], row[2]-stats.μ[0], row[3]-stats.μ[1]] )
+}
+
+function outliers (data, sd=1.96) {
+	const stats = computeStats(data);
+
+	function fn ([l, p, i, j]) {
+		return (i - stats.μ[0]) > stats.σ[0]*sd ||
+			(j - stats.μ[1]) > stats.σ[1]*sd;
+	}
+
+	return data.filter(fn)
+}
+
+function inliers (data, sd=1.96) {
+	const stats = computeStats(data);
+
+	function fn ([l, p, i, j]) {
+		return (i - stats.μ[0]) <= stats.σ[0]*sd &&
+			(j - stats.μ[1]) <= stats.σ[1]*sd;
+	}
+
+	return data.filter(fn)
+}
+
+function difference (a, b) {
+	const obj = Array.isArray(a) ? [] : {};
+	for (const k in a) {
+		const v0 = a[k];
+		const v1 = b[k]
+		if (v0 instanceof Object && v1 instanceof Object) {
+			obj[k] = difference (v0, v1);
+		} else if (!isNaN(Number(v0)) && !isNaN(Number(v1))) {
+			obj[k] = v1 - v0;
+		}
+	}
+	return obj;
+}
+
+function combinations (a, n) {
+	const results = [];
+
+	function combine(current, start) {
+		if (current.length === n) {
+			results.push([...current]);
+			return;
+		}
+		for (let i = start; i < a.length; i++) {
+			current.push(a[i]);
+			combine(current, i + 1);
+			current.pop();
+		}
+	}
+
+	combine([], 0);
+	return results;
+}
+
+module.exports = {
+	combinations,
+	centre,
+	ijRMS,
+	computeStats,
+	computeSample,
+	stratifiedSample,
+	old_stratifiedSample,
+	decimate,
+	difference,
+	outliers,
+	inliers
+}
diff --git a/lib/www/server/package.json b/lib/www/server/package.json
index 9a7a7dd..e850b4d 100644
--- a/lib/www/server/package.json
+++ b/lib/www/server/package.json
@@ -44,6 +44,7 @@
     "jsonwebtoken": "^9.0.2",
     "leaflet-headless": "git+https://git@gitlab.com/aaltronav/contrib/leaflet-headless.git#devel",
     "marked": "^4.0.12",
+    "mathjs": "^14.6.0",
     "node-fetch": "^2.6.1",
     "nunjucks": "^3.2.3",
     "path-to-regexp": "^6.2.1",
diff --git a/package-lock.json b/package-lock.json
index 66072dd..b31979b 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1678,17 +1678,6 @@
       "dev": true,
       "license": "MIT"
     },
-    "lib/www/client/source/node_modules/@babel/runtime": {
-      "version": "7.23.2",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "regenerator-runtime": "^0.14.0"
-      },
-      "engines": {
-        "node": ">=6.9.0"
-      }
-    },
     "lib/www/client/source/node_modules/@babel/template": {
       "version": "7.27.2",
       "dev": true,
@@ -7524,11 +7513,6 @@
         "node": ">=4"
       }
     },
-    "lib/www/client/source/node_modules/regenerator-runtime": {
-      "version": "0.14.0",
-      "dev": true,
-      "license": "MIT"
-    },
     "lib/www/client/source/node_modules/regenerator-transform": {
       "version": "0.15.2",
       "dev": true,
@@ -9374,6 +9358,7 @@
         "jsonwebtoken": "^9.0.2",
         "leaflet-headless": "git+https://git@gitlab.com/aaltronav/contrib/leaflet-headless.git#devel",
         "marked": "^4.0.12",
+        "mathjs": "^14.6.0",
         "node-fetch": "^2.6.1",
         "nunjucks": "^3.2.3",
         "path-to-regexp": "^6.2.1",
@@ -10527,17 +10512,6 @@
         "node": ">=6.0.0"
       }
     },
-    "lib/www/server/node_modules/redoc-cli/node_modules/@babel/runtime": {
-      "version": "7.16.7",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "regenerator-runtime": "^0.13.4"
-      },
-      "engines": {
-        "node": ">=6.9.0"
-      }
-    },
     "lib/www/server/node_modules/redoc-cli/node_modules/@babel/template": {
       "version": "7.12.13",
       "dev": true,
@@ -12532,11 +12506,6 @@
         "url": "https://github.com/Mermade/oas-kit?sponsor=1"
       }
     },
-    "lib/www/server/node_modules/redoc-cli/node_modules/regenerator-runtime": {
-      "version": "0.13.9",
-      "dev": true,
-      "license": "MIT"
-    },
     "lib/www/server/node_modules/redoc-cli/node_modules/require-directory": {
       "version": "2.1.1",
       "dev": true,
@@ -13297,6 +13266,15 @@
         "node": ">=0.4"
       }
     },
+    "node_modules/@babel/runtime": {
+      "version": "7.28.3",
+      "resolved": "https://registry.npmjs.org/@babel/runtime/-/runtime-7.28.3.tgz",
+      "integrity": "sha512-9uIQ10o0WGdpP6GDhXcdOJPJuDgFtIDtN/9+ArJQ2NAfAmiuhTQdzkaTGR33v43GYS2UrSA0eX2pPPHoFVvpxA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6.9.0"
+      }
+    },
     "node_modules/@deck.gl/aggregation-layers": {
       "version": "9.1.13",
       "resolved": "https://registry.npmjs.org/@deck.gl/aggregation-layers/-/aggregation-layers-9.1.13.tgz",
@@ -14389,6 +14367,19 @@
         "node": ">= 10"
       }
     },
+    "node_modules/complex.js": {
+      "version": "2.4.2",
+      "resolved": "https://registry.npmjs.org/complex.js/-/complex.js-2.4.2.tgz",
+      "integrity": "sha512-qtx7HRhPGSCBtGiST4/WGHuW+zeaND/6Ld+db6PbrulIB1i2Ev/2UPiqcmpQNPSyfBKraC0EOvOKCB5dGZKt3g==",
+      "license": "MIT",
+      "engines": {
+        "node": "*"
+      },
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/rawify"
+      }
+    },
     "node_modules/compressible": {
       "version": "2.0.18",
       "resolved": "https://registry.npmjs.org/compressible/-/compressible-2.0.18.tgz",
@@ -15089,6 +15080,12 @@
         "node": ">= 0.4"
       }
     },
+    "node_modules/escape-latex": {
+      "version": "1.2.0",
+      "resolved": "https://registry.npmjs.org/escape-latex/-/escape-latex-1.2.0.tgz",
+      "integrity": "sha512-nV5aVWW1K0wEiUIEdZ4erkGGH8mDxGyxSeqPzRNtWP7ataw+/olFObw7hujFWlVjNsaDFw5VZ5NzVSIqRgfTiw==",
+      "license": "MIT"
+    },
     "node_modules/escodegen": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/escodegen/-/escodegen-2.1.0.tgz",
@@ -15205,6 +15202,19 @@
         "node": ">= 6"
       }
     },
+    "node_modules/fraction.js": {
+      "version": "5.3.1",
+      "resolved": "https://registry.npmjs.org/fraction.js/-/fraction.js-5.3.1.tgz",
+      "integrity": "sha512-PhqCuhSKIGbbkJ+cojHv47eEWClU71FIOhiUsYdZYTwhIzCeIN8rXeEjserTvPat5JLJChumn8chHz64WkZgTw==",
+      "license": "MIT",
+      "engines": {
+        "node": "*"
+      },
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/rawify"
+      }
+    },
     "node_modules/fs-minipass": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/fs-minipass/-/fs-minipass-2.1.0.tgz",
@@ -15635,6 +15645,12 @@
       "resolved": "https://registry.npmjs.org/isstream/-/isstream-0.1.2.tgz",
       "integrity": "sha512-Yljz7ffyPbrLpLngrMtZ7NduUgVvi6wG9RJ9IUcyCd59YQ911PBJphODUcbOVbqYfxe1wuYf/LJ8PauMRwsM/g=="
     },
+    "node_modules/javascript-natural-sort": {
+      "version": "0.7.1",
+      "resolved": "https://registry.npmjs.org/javascript-natural-sort/-/javascript-natural-sort-0.7.1.tgz",
+      "integrity": "sha512-nO6jcEfZWQXDhOiBtG2KvKyEptz7RVbpGP4vTD2hLBdmNQSsCiicO2Ioinv6UI4y9ukqnBpy+XZ9H6uLNgJTlw==",
+      "license": "MIT"
+    },
     "node_modules/jsbn": {
       "version": "0.1.1",
       "resolved": "https://registry.npmjs.org/jsbn/-/jsbn-0.1.1.tgz",
@@ -15848,6 +15864,29 @@
         "node": ">= 0.4"
       }
     },
+    "node_modules/mathjs": {
+      "version": "14.6.0",
+      "resolved": "https://registry.npmjs.org/mathjs/-/mathjs-14.6.0.tgz",
+      "integrity": "sha512-5vI2BLB5GKQmiSK9BH6hVkZ+GgqpdnOgEfmHl7mqVmdQObLynr63KueyYYLCQMzj66q69mV2XZZGQqqxeftQbA==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@babel/runtime": "^7.26.10",
+        "complex.js": "^2.2.5",
+        "decimal.js": "^10.4.3",
+        "escape-latex": "^1.2.0",
+        "fraction.js": "^5.2.1",
+        "javascript-natural-sort": "^0.7.1",
+        "seedrandom": "^3.0.5",
+        "tiny-emitter": "^2.1.0",
+        "typed-function": "^4.2.1"
+      },
+      "bin": {
+        "mathjs": "bin/cli.js"
+      },
+      "engines": {
+        "node": ">= 18"
+      }
+    },
     "node_modules/md5": {
       "version": "2.3.0",
       "resolved": "https://registry.npmjs.org/md5/-/md5-2.3.0.tgz",
@@ -16343,6 +16382,12 @@
         "node": ">=10"
       }
     },
+    "node_modules/seedrandom": {
+      "version": "3.0.5",
+      "resolved": "https://registry.npmjs.org/seedrandom/-/seedrandom-3.0.5.tgz",
+      "integrity": "sha512-8OwmbklUNzwezjGInmZ+2clQmExQPvomqjL7LFqOYqtmuxRgQYqOD3mHaU+MvZn5FLUeVxVfQjwLZW/n/JFuqg==",
+      "license": "MIT"
+    },
     "node_modules/semver": {
       "version": "7.7.2",
       "resolved": "https://registry.npmjs.org/semver/-/semver-7.7.2.tgz",
@@ -16606,6 +16651,12 @@
         "texture-compressor": "bin/texture-compressor.js"
       }
     },
+    "node_modules/tiny-emitter": {
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/tiny-emitter/-/tiny-emitter-2.1.0.tgz",
+      "integrity": "sha512-NB6Dk1A9xgQPMoGqC5CVXn123gWyte215ONT5Pp5a0yt4nlEoO1ZWeCwpncaekPHXO60i47ihFnZPiRPjRMq4Q==",
+      "license": "MIT"
+    },
     "node_modules/toidentifier": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/toidentifier/-/toidentifier-1.0.1.tgz",
@@ -16667,6 +16718,15 @@
         "node": ">= 0.6"
       }
     },
+    "node_modules/typed-function": {
+      "version": "4.2.1",
+      "resolved": "https://registry.npmjs.org/typed-function/-/typed-function-4.2.1.tgz",
+      "integrity": "sha512-EGjWssW7Tsk4DGfE+5yluuljS1OGYWiI1J6e8puZz9nTMM51Oug8CD5Zo4gWMsOhq5BI+1bF+rWTm4Vbj3ivRA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 18"
+      }
+    },
     "node_modules/undici-types": {
       "version": "7.8.0",
       "resolved": "https://registry.npmjs.org/undici-types/-/undici-types-7.8.0.tgz",