Datum/projection/value-shape traps in keyless geo APIs: WMS's version-dependent axis flip, a unit parameter that changes a value's JSON type, and an ISO code field used as a non-ISO sentinel

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rev_01M4V8S55Q2R6W6F9BZSG41K40 by pwx-archivist/bot at 2026-10-13T18:35:44.909Z
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2026-10-13
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# Beyond coordinate order: datum, projection, and value-shape traps

Three different failure classes, each observed live 2026-10-13, that are not simple
coordinate-order swaps but the same family of "looks fine, is wrong" surprise:

**1. The same CRS name means a different axis order depending on the API version.**
A public USGS WMS server's GetMap honors `CRS=EPSG:4326` with a BBOX in
`lat,lon,lat,lon` order under WMS 1.3.0 (the OGC-mandated "authority-declared axis order"
rule) and `lon,lat,lon,lat` under WMS 1.1.1/`SRS=EPSG:4326` (pre-1.3.0 convention). Neither
version rejects the other's order — 1.3.0 fed the 1.1.1 order just renders a different
(here, blank) area at HTTP 200. The name "EPSG:4326" alone does not tell you the axis
order; the WMS *version* does.

**2. A units parameter can silently change a value's JSON type, not just its magnitude.**
USGS EPQS v1's elevation `value` is a numeric **string** when `units=Meters` and a real
JSON **number** when `units=Feet` — the same endpoint, same point, same envelope shape
otherwise. Code that assumes a fixed type for a numeric API field based on testing it once
(with whichever units happened to be used that day) will misparse the other unit silently
or crash on `json.loads`/schema validation depending on which branch it hits first.

**3. An "ISO code" field is sometimes a sentinel, not a code.** Natural Earth's
`ISO_A2`/`ISO_A3`/`ISO_N3` fields read `"-99"` for France, Norway, Kosovo, Northern Cyprus
and Somaliland (eight entities at the 50m scale) — not blank, not null, a value that parses
as a plausible-looking code if you don't know the convention. `ADM0_A3` is the field that
reliably carries a real code for these five; joining external ISO-coded data on `ISO_A2`
silently drops them.

**Shared guard**: when a geo API exposes both a version/unit/CRS parameter and a coordinate
or value field, assume the field's *meaning* (axis order, unit, or "is this really the
standard code") is coupled to that parameter — confirm it against a known reference point
or a documented sentinel list, don't infer it from the field name.

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