Terminology
Terms are defined as Terra uses them. Where a word carries a different sense elsewhere in geospatial work, that is noted.
Grid structure
Cell — One hexagon of the Terra grid at a given resolution. The unit the whole system addresses. A cell is a two-dimensional region of ground; it has no height extent.
Resolution — One of the 15 levels of the grid, Res 0 (coarsest) through Res 14 (finest). Each
finer level divides area by seven. Written Res 8, or 08 inside a Terra Grid Code.
Note the direction: Res 0 is the largest cell, Res 14 the smallest. Systems that number levels the other way exist, and mixing the two conventions in one document is a recurring source of error.
Res 8 — The national base standard. Cells of exactly one hectare. The resolution at which Terra is intended to be used unless there is a specific reason to go coarser or finer.
Base cell — One of the coarsest cells, at Res 0, from which all finer cells descend. The base cells and their anchor positions are fixed by the specification.
Child digit — A value 0–6 recording which of a parent's seven children was taken at one level of descent. Digit 0 is the centre child; 1–6 are the surrounding ring. Digits below a cell's resolution are set to the unused marker, 7.
Aperture — The ratio between successive resolutions. Terra is aperture 7: seven children per parent, area divided by seven, edge length divided by √7.
Identifiers
Terra index — The canonical identity of a cell: an unsigned 64-bit integer encoding mode, resolution, base cell, and child digits.
TIS — Terra Index String — The index written as t followed by 16 hexadecimal characters.
Always 17 characters. For code, databases, and APIs.
TGC — Terra Grid Code — The human-readable form, T[RES]-[POSITION][CHECK]. Variable length,
7 to 15 characters. For people.
Check character — The final character of a TGC, a Luhn mod 32 value over the position body. Not part of the position; validated and discarded.
Unused marker — The value 7 in a child digit position, indicating that digit lies below the cell's resolution. Distinguishes an unused digit from a legitimate centre child, which is 0.
Relationships
Parent — The cell at the next coarser resolution containing a given cell, derived by arithmetic on the index.
Child — One of the seven cells at the next finer resolution descending from a given cell.
Centre child — The child whose digit is 0, positioned at the parent's centre.
Ancestor / descendant — Parent and child extended across more than one level.
Neighbour — One of the six cells sharing an edge with a given cell. Every Terra cell has exactly six, at every resolution, without exception.
Grid distance — The number of steps between two cells, moving neighbour to neighbour. Distinct from metric distance, which is measured in metres.
Grid disk — All cells within grid distance k of an origin, filled. Contains 3k² + 3k + 1 cells.
Grid ring — Only the cells at exactly grid distance k. Contains 6k cells for k ≥ 1.
Geometry
LAEA — Lambert Azimuthal Equal Area, the projection Terra is laid out on, centred 24°N 45°E. Preserves area exactly; shape and distance distort with distance from the centre.
Equal-area — The property that a cell covers the same ground area anywhere in the coverage extent. This is why counting cells is equivalent to measuring area.
Boundary — The vertices of a cell's outline, returned in order.
Centre — The centroid of a cell, the position a cell resolves to when converted back to a coordinate.
Approximate nesting — The property that a parent hexagon is not exactly the union of its seven children, because the child grid is rotated ~19.1° from the parent. Areas and counts roll up exactly; footprints differ slightly near boundaries. See How Terra works.
Reference frame
SANSRS — The Saudi National Spatial Reference System, the national standard Terra aligns to.
KSA-GRF17 — The horizontal geodetic datum, EPSG:9333. Every Terra coordinate is interpreted on it.
KSA-VRF14 / KSA-GEOID21 / KSA-CORS — The vertical frame, geoid model, and reference station network. See SANSRS integration.
Operations
Indexing — Converting a coordinate to the cell containing it.
Compaction — Replacing a set of cells with the smallest mixed-resolution set covering the same ground, by collapsing complete sibling groups into their parent.
Uncompaction — The reverse: expanding a mixed-resolution set to a uniform resolution.
Partial cell — A cell only partly inside a region of interest, such as one straddling a coastline or an administrative boundary. Carried with a coverage fraction rather than being included or excluded outright.
The Terra System is designed and developed by Tec Solution KSA.