SANSRS integration
Terra is defined on the national reference frame, not on a generic global one. A coordinate handed to Terra is interpreted as KSA-GRF17, and cells are laid out on a projection anchored to that datum. This page covers what that means in practice, and what to do with data that arrives on something else.
The reference frame
The Saudi National Spatial Reference System (SANSRS) is the national standard, mandated by Council of Ministers Decision No. 90. Its components:
| Component | Role | Terra's use |
|---|---|---|
| KSA-GRF17 | Horizontal geodetic datum, EPSG:9333, GRS 1980 ellipsoid | The datum every Terra coordinate is interpreted on |
| KSA-VRF14 | National vertical reference frame | Terra is two-dimensional; heights pass through untouched |
| KSA-GEOID21 | Geoid model relating ellipsoidal to orthometric height | Used only when converting heights alongside Terra work |
| KSA-CORS | Continuously Operating Reference Station network | The source of survey-grade positions on the frame |
Terra indexes horizontal position only. Elevation, depth, and floor level are attributes of your data, not of the cell. A cell is a column of ground, and everything above and below it shares one identifier.
Why the datum matters
A datum mismatch does not raise an error. It silently returns the wrong cell.
WGS 84 and KSA-GRF17 are both GRS 1980-based and appear interchangeable in casual use. They are not identical, and the offset between them is on the order of metres — which is immaterial at Res 4, marginal at Res 8, and decisive at Res 11 and finer.
| Resolution | Cell width | Effect of a metre-scale datum offset |
|---|---|---|
| 4 | ~5.2 km | negligible |
| 8 | ~107 m | occasionally shifts a point across a boundary |
| 11 | ~5.8 m | routinely wrong cell |
| 14 | ~0.31 m | always wrong cell |
The practical rule: at Res 8 a datum error is an occasional boundary defect; at Res 11 and finer it is a systematic one. Anyone indexing at fine resolutions must confirm the datum of their source before trusting the result.
Working with incoming data
Transform first, index second. Terra performs no datum transformation. It assumes what it is given is already on KSA-GRF17.
| Source | Action |
|---|---|
| KSA-GRF17 / EPSG:9333 | Index directly |
| GNSS observation via KSA-CORS | Already on the national frame; index directly |
| WGS 84 / EPSG:4326 (consumer GPS, web maps) | Transform to KSA-GRF17 first |
| Ain Al Abd 1970 and other legacy frames | Transform; see below |
| Unknown datum | Establish it before indexing. Do not assume |
Transformation belongs in your ingest pipeline, using the transformation parameters published by the national geospatial authority — not in Terra, and not improvised.
Legacy frame migration
Datasets predating SANSRS are commonly held on Ain Al Abd 1970 or on project-specific local frames. Offsets from KSA-GRF17 can reach hundreds of metres, which is many cells wide at any resolution anyone would use operationally.
A migration that indexes legacy coordinates without transforming them produces cells that look entirely plausible — correctly formed, in the right country, near the right city — and are simply wrong. Nothing downstream will detect it.
Recommended sequence:
- Establish the source frame from the dataset's own metadata or its survey record. Do not infer it from where the points appear to fall.
- Transform to KSA-GRF17 using published parameters for that frame.
- Validate on known control — points whose position on the national frame is independently known — before indexing the bulk.
- Index, and record the source frame and transformation used as dataset metadata.
Step 4 matters more than it appears. A dataset carrying its provenance can be re-derived if a transformation is later found to be wrong. One that does not cannot be distinguished from a correct dataset by inspection.
Accuracy and resolution
Choose a resolution your positions can support. A Terra code carries the precision its resolution implies, and a Res 12 code asserts sub-metre accuracy whether or not the underlying fix justifies it.
| Position source | Typical horizontal accuracy | Defensible resolution |
|---|---|---|
| KSA-CORS RTK survey | centimetres | Res 12–14 |
| Differential GNSS | sub-metre | Res 11 |
| Consumer GNSS (phone, vehicle) | 3–10 m | Res 8–9 |
| Digitised from imagery | varies with scale | Res 8 or coarser |
| Geocoded from an address | tens of metres | Res 6–8 |
Publishing at a finer resolution than the source supports transfers false confidence to everyone downstream, who cannot see where the number came from.
Reference
- Datum: KSA-GRF17, EPSG:9333, GRS 1980 ellipsoid
- Projection: Lambert Azimuthal Equal Area, centred 24°N 45°E
- Vertical: not used by Terra; heights are dataset attributes
- Authority: SANSRS, per Council of Ministers Decision No. 90
For transformation parameters, the CORS network, and the current geoid model, consult the national geospatial authority directly. Terra does not redistribute these, since republished geodetic parameters go stale and a stale parameter is worse than an absent one.
Next
- How Terra works — the projection and grid layout
- Tables of cell stats — precision by resolution
The Terra System is designed and developed by Tec Solution KSA.