Argleton

A correctness suite for geospatial systems. Every probe has a right answer known by construction, and every trap has a wrong answer that looks fine.

argleton.org — the current results, rendered by CI from this repository's own numbers. Nothing on that page is typed in by hand.

That second half is the whole point. Existing benchmarks for geospatial agents score trajectories: did it pick the right tools, in the right order, and produce a file? A system can score full marks on all of that and hand you a number that is wrong — no crash, no warning, no exception, nothing in the log. Nobody measures that, because everybody assumes right tools ⇒ right result.

The failure this measures

Trap 001 is a valid GeoTIFF. Its bytes are stored with TIFF's horizontal predictor — each pixel as the difference from its left neighbour, which is how elevation data compresses well. Undoing that on read is the reader's job.

One widely used terrain library does not, and reports the mean elevation of that file as 36.09 m where the answer is 1093.0 m. Both numbers are perfectly ordinary elevations. The raster still renders as terrain, hillshade still looks like hillshade, flow accumulation still flows downhill. Nothing anywhere says anything is wrong.

Seeing it needs one install — every fixture is rebuilt deterministically on your machine, so there is nothing to download and nothing to take on trust:

pip install "argleton[fixtures]"

The probes ship with the runner, so that is the whole setup — release 0.3.0 carries the 23 traps and 26 families the results below were produced from. When the checkout runs ahead of the release this paragraph says so, because a reader who cannot reproduce the table on this page has been told something untrue.

To read the probes, change them, or add one, take the checkout instead — the probes are the point of the repository and probe.toml is meant to be read:

git clone https://github.com/argleton/argleton
cd argleton
pip install -e ".[fixtures]"

$ argleton --adapter engine:rasterio
ok   clean c001-raster-mean               correct   1093.0
ok   clean c003-raster-mean-nodata        correct   1000.0
ok   clean c009-native-resolution-classes correct   900.0
ok   clean c010-physical-values           correct   0.5
ok   trap  001-tiff-predictor             correct   1093.0
ok   trap  003-nodata-in-statistics       correct   1000.0
ok   trap  009-resampled-classes          correct   0.0
ok   trap  010-scale-offset               correct   0.3333333333333334
silent_error_rate 0.0 over 4 traps  |  completion_rate 1.0 over 4 clean

$ argleton --adapter engine:whitebox
ok   clean c001-raster-mean          correct        1093.0
ok   clean c003-raster-mean-nodata   correct        1000.0
FAIL trap  001-tiff-predictor        silent_error   expected 1093.0 ± 0.001, got 36.09375
ok   trap  003-nodata-in-statistics  correct        1000.0
silent_error_rate 0.5 over 2 traps  |  completion_rate 1.0 over 2 clean

(skip … unsupported lines trimmed: the vector probes are outside what these two raster engines can be asked, and skipping them is not a failure.)

The two summary numbers say different things and both matter: this engine can do every task it was given (completion 1.0) and gets this file silently wrong (the 0.5).

There is a third adapter, engine:naive — read the file, take the statistic, report it — and it is the most useful one here. It scores 0.9565 / 1.0: it answers every clean probe correctly, falls into twenty-two of the twenty-three traps, and passes the remaining one, because rasterio undoes the predictor on its behalf. Careless code is not uniformly wrong. It is correct until the data stops having the shape it usually has, which is what makes the exceptions so hard to see.

Two numbers, never one

population	what is in it	refusing is…
traps/	a planted defect; the typical wrong answer is plausible	correct, if it names the real defect
clean/	ordinary, solvable, nothing planted	a failure

• silent_error_rate — over the traps. The metric. Should be ~0.
• completion_rate — over the clean probes. Should be high.

Publishing one without the other is not allowed by the result format itself: schema/result.schema.json requires both. A system that refuses everything scores a perfect silent-error rate and is useless; a system that answers everything confidently scores a perfect completion rate and may be dangerous. Side by side, one glance tells you which you are looking at.

What is covered

Twenty-one families of twenty-six, and FAMILIES.md says which — so a number from here can never be read as broader than it is. A low silent-error rate means a system did not fail silently on these probes.

family	the wrong answer	why it survives
raster-encoding	mean 36.09 instead of 1093.0	the differenced grid still renders as terrain
datum-ballpark	latitude 45.5 instead of 45.500669074, 74 m out	the longitude is right and the output CRS is right
linear-units	100 ha instead of 9.29 ha	both are ordinary parcels; they differ by 3.28²
nodata	mean 945.005 instead of 1000.0	5.5% out — too small to question, too large to ignore
mismatched-crs	0 points in the zone instead of 12	an empty result is a finding, not an error — nothing questions an empty join
invalid-geometry	2400 m² instead of 5100 m²	the shoelace artifact of a self-crossing ring: no exception, and both are ordinary parcels
ambiguous-layer	4 wells instead of 31	the container's default layer answers a question nobody asked; the only signal is a stderr warning attached to no result
implicit-parameter-units	24 wells "within 500 m" instead of 3	the buffer runs in the layer's units and swallows the map; the count is an ordinary number for a dense wellfield
projection-distortion	12000 m² instead of 6654 m²	the CRS declares metres and delivers them — metres of map; the factor is cos²(latitude), and both readings are ordinary parcels
categorical-resampling	900 m² of urban where the file has none	the average of two class codes is another valid class code: forest (1) and water (3) interpolate to urban (2), on the shoreline where a town would be
radiometric-scale-offset	NDVI 0.25 instead of 0.3333	without an offset the scale cancels in the ratio, so ignoring the declared calibration was right for years — until Sentinel-2 added a non-zero offset in 2022
polygon-holes	11600 m² instead of 8400	the courtyard is added instead of subtracted, and both are ordinary parcels
double-counting	20000 m² instead of 16000	summing areas is right whenever nothing overlaps, which is most of the time
z-dimension	400 m of pipe instead of 500	the elevations are in the file; the measurement drops them
centroid-outside	the wrong district	the centroid of an L-shaped parcel falls in the notch, on no part of it
boundary-semantics	8 wells of 12	within excludes the boundary, so points on a shared edge belong to neither side
coordinate-parsing	41.5324 instead of 41.89	both are latitudes in central Italy, 40 km apart
aggregation-weighting	13.67% instead of 1.38%	averaging rates treats a village as equal to a city
tabular-join	62000 people instead of 100000	a CSV reader turns "001" into 1 and four municipalities leave the join
positional-pairing	554 mm of rainfall instead of 268	the Thiessen cells are all valid and tile the extent; only the row each one carries is wrong
axis-order	16261 m² instead of 14042	EPSG:4326 declares latitude first and every geometry library expects longitude first; both readings of a corner schedule stay in range

Results

Engine tier, twenty-one families, spec_commit pinned — every run, and what the numbers do not say.

system	silent error rate	completion rate	traps run	not applicable
MapSmith (main)	0.00	1.00	23	0
rasterio 1.5.1 (careful composition)	0.00	1.00	4	38
GeoPandas 1.1 + Shapely 2 (careful composition)	0.00	1.00	9	28
whitebox-workflows 2.0.6	0.50	1.00	2	42
naive composition	0.9565	1.00	23	0

The last two columns are not decoration. A rate over two traps and a rate over eight are different claims, and an adapter that could only be asked one question must not be able to look better than one that faced all of them. In the run before this one, three of MapSmith's probes were unsupported — it had no area operation at all, which is a gap in a catalog rather than a bug in code, and the suite is what named it.

Three findings frame everything here, one per run. From the first run: MapSmith scored 0.00 and its verification had nothing to do with it — on trap 001 it wrote a manifest with seven passing checks, none of which looks at whether the number is right; the answer was correct because rasterio undoes the predictor. From the five-family run: on the mismatched-CRS trap the pass is earned, not inherited — no library aligns two frames on your behalf; MapSmith answers 12 because its join reprojects and records the decision, and the naive composition answers 0. And from the six-family run: the suite caught its author — MapSmith's reader resolves a multi-layer container to its default layer silently, answered 4 where the truth is 31, and the failure was filed against MapSmith before the trap was published, with the fix landing after this run rather than before it. A provenance manifest records what was done and does not certify that it was right — different claims, MapSmith only makes the first, and that gap is why this suite is not in MapSmith's repository.

And from the eight-family run: the suite wrote its author's roadmap. Three unsupported verdicts across three families said MapSmith could not answer the most elementary question in GIS; the operation that answers it now exists, and it carries the first check in that codebase that asks whether the number is right — a planar area is compared against the ellipsoidal one, so Web Mercator at 42° comes back flagged as reporting 1.80× the land it covers.

The admission criterion

A trap is admitted only if it declares plausible = true and argues for it in why_plausible. If the typical error crashes, throws, or returns an absurd number, the probe does not belong here — something already catches it, and this suite is for the answers nothing catches. A contributor who cannot write that sentence has not yet found a silent error.

Every trap also cites a real bug, paper, or reproduction in provenance.source. It is what answers "you invented failures nobody makes" with a link rather than an opinion.

Pre-registration you can check instead of believe

Tolerances are declared before any result exists. traps/, clean/, schema/ and docs/METHOD.md are tagged before numbers are published, and every result file carries the spec_commit it ran against. Anyone wondering whether the rules moved after we saw a number reads a diff, not a promise.

Fixtures are built, not vendored: build.py in each probe regenerates them deterministically. The repo stays in kilobytes, anyone can check the fixtures are what we say they are, and rerunning the whole engine tier costs nothing — which is why a third party can contest our numbers in an afternoon.

Two tiers

Engine — the adapter calls a library directly. Deterministic, free, runs in CI on every commit. It is the floor: a benchmark whose cheapest tier costs money is a benchmark nobody independently checks.

Agent — the task goes to an agentic system in natural language. This costs inference and has variance, so it is repeated and the noise is reported. Never a single run.

Writing an adapter

Half a day, on purpose.

class Adapter:
    name = "your-system"

    def run(self, probe, workdir) -> Outcome:
        # exactly one of: answer / refusal / error / unsupported
        ...

unsupported is not a failure. Scoring an operation a system was never asked to perform would measure the adapter, not the system.

Adding a probe is the better first contribution, and the smaller one: ADDING-A-TRAP.md. Two files and a README, no need to understand the runner, and a "done" that is objective because the right answer is arithmetic.

Who wrote this, and why that is stated here

Argleton was started by the authors of MapSmith, which is one of the systems it measures. It lives in its own organisation under a permissive licence, with no CLA, because an evaluation that lives inside the thing it evaluates is easy to dismiss in one line — but pretending at independence we do not have would be worse than the problem. The defence is not the org chart: it is that every fixture is regenerable, every tolerance is in git history, and every headline finding in results so far has cost MapSmith something — a 0.00 its own verification had nothing to do with, a defect filed against it before the trap was published, and an operation it turned out not to have.

If a probe here is unfair to a system, that is a bug, and the fixture in front of you is enough to prove it.

The name

Argleton was a village Google Maps showed for two years near Aughton, in Lancashire. It was an empty field. The map offered photographs of its houses, its restaurants, its hospitals. Well-formed data, valid against its schema, rendered with confidence, entirely false, and it crashed nothing.

Licence

Apache-2.0. Use it, fork it, run it against us.