.. _library_geohash:

`geohash`

The geohash library provides a geohash_protocol protocol and a geohash object for encoding and decoding geographic coordinates using the standard geohash algorithm.

It uses the standard geohash base-32 alphabet:

0123456789bcdefghjkmnpqrstuvwxyz The library complements the geospatial library and uses the same geographic(Latitude,Longitude) coordinate representation and bbox(geographic(MinLatitude,MinLongitude),geographic(MaxLatitude,MaxLongitude)) bounding-box representation.
For encoding, longitudes are canonicalized to the [-180.0,180.0[ range so that geographic(Latitude,180.0) and geographic(Latitude,-180.0) encode to the same geohash.

API documentation

Open the `../../apis/library_index.html#geohash <../../apis/library_index.html#geohash>`__ link in a web browser.

Loading

To load all entities in this library, load the loader.lgt file:

| ?- logtalk_load(geohash(loader)).

Testing

To test this library predicates, load the tester.lgt file:

| ?- logtalk_load(geohash(tester)).

Supported operations

The library provides predicates for:

validating geohash atoms
encoding coordinates at a requested precision
decoding a geohash to its cell center
decoding a geohash to its cell bounding box
querying the nominal latitude and longitude precision of a geohash length
querying cell dimensions for a geohash length
computing adjacent and neighboring cells
covering a geographic bounding box with geohashes of a fixed precision
covering a geographic bounding box using fixed-depth or adaptive mixed-depth covers
compressing complete sibling sets into parent prefixes
navigating geohash parent-child relationships and shared prefixes
converting between atom geohashes and integer-backed 5-bit packed representations
converting between atom and integer geohash representations explicitly
encoding and decoding integer geohashes using bit precision
expanding a geohash to the center cell plus neighbors
covering polygons and polylines with geohashes

Examples

Encode a coordinate at a selected precision:

| ?- geohash::encode(geographic(42.6, -5.6), 5, Geohash). Geohash = 'ezs42' yes

| ?- geohash::encode(geographic(57.64911, 10.40744), 11, Geohash). Geohash = 'u4pruydqqvj' yes

Decode a geohash to its center coordinate and bounding box:

| ?- geohash::decode('ezs42', Coordinate). Coordinate = geographic(42.60498046875, -5.60302734375) yes

| ?- geohash::bounding_box('ezs42', BoundingBox). BoundingBox = bbox(geographic(42.5830078125, -5.625), geographic(42.626953125, -5.5810546875)) yes

Inspect cell precision metadata:

| ?- geohash::precision(5, LatitudeError, LongitudeError). | ?- geohash::cell_dimensions(5, LatitudeSpan, LongitudeSpan).

Compute adjacent cells and neighbors:

| ?- geohash::adjacent('ezs42', north, North). | ?- geohash::neighbors('ezs42', Neighbors).

Cover a bounding box with geohashes of a fixed precision:

| ?- geohash::covering(bbox(geographic(42.58, -5.63), geographic(42.63, -5.58)), 5, Geohashes).

Compute adaptive and compact covers:

| ?- geohash::covering(bbox(geographic(42.58, -5.63), geographic(42.63, -5.58)), max_precision(6), Geohashes, []).

| ?- geohash::covering(bbox(geographic(42.58, -5.63), geographic(42.63, -5.58)), precision(6), Geohashes, [compact(true), min_precision(4)]).

Inspect hierarchy and integer representations:

| ?- geohash::parent('ezs42', Parent).

| ?- geohash::children('ezs4', Children).

| ?- geohash::encode_int(geographic(42.6, -5.6), 5, HashInteger).

| ?- geohash::bounding_box_int(HashInteger, 5, BoundingBox).

| ?- geohash::geohash_to_int('ezs42', HashInteger).

| ?- geohash::int_to_geohash(HashInteger, 5, Geohash).

| ?- geohash::encode_bits(geographic(42.6, -5.6), 25, HashInteger).

| ?- geohash::expand('ezs42', ExpandedGeohashes).

Cover polygons and polylines:

Notes

Geohash cell width and height alternate between equal and unequal angular resolution depending on whether the geohash length is even or odd.
Encoding canonicalizes longitude 180.0 to -180.0 so dateline-equivalent coordinates map to the same hash.
Neighbor and covering computations use decoded cell dimensions and therefore handle antimeridian wrapping for eastward and westward movement.
Northward adjacency from the northernmost cells and southward adjacency from the southernmost cells fails because there is no cell beyond the poles.
Bounding-box covering accepts antimeridian-crossing boxes by allowing the minimum longitude to be greater than the maximum longitude.
Degenerate point bounding boxes are accepted by covering/3 and return the single geohash cell containing the point.
covering/4 keeps covering/3 as the fixed-precision shorthand while adding a CoverSpec argument with precision(Precision) and max_precision(MaxPrecision) forms.
geohash now imports the shared options library and uses it to validate and merge covering options.
covering/4 and polygon_covering/4 accept compact(Boolean) and min_precision(PositiveInteger) options. Compact covers never compress below the selected minimum precision.
polyline_covering/4 also accepts a buffer(Distance) option where the distance is expressed in kilometers.
compress/2 operates on complete sibling sets only, replacing them with parent prefixes while preserving the covered area.
Integer-backed predicates mirror the string predicates and keep the precision argument explicit, avoiding any hidden dependence on integer bit width.
Bit-precision predicates provide the same packed integer representation while measuring precision directly in bits instead of characters.
polygon_covering/4 and polyline_covering/4 rely on generic geospatial geometry predicates and currently do not support antimeridian-crossing input geometries.

geohash