geoid – Efficient storage and queriying of geographic data

The geoid library allows representing latidude/longitude coordinates using 64-bit integers. The integer values are ordered to preseve locality, which means that these IDs can be stored in databases and used to query and efficiently retrieve locations which are in a geographic region.

This library is inspired by the S2 Geometry library, in the sense that the integer IDs are determined in the same way. Until this libary has its own introduction section, you can read S2 Cell Hierarchy to understand how geoids work. What this library calls geoids, are called cells by the S2 libary.

Also, while inspired by the S2 library, this Racket module is an independent implementation that does not generate compatible IDs and does not aim to have the same API and functionality as that library.

• A geoid is a 64 bit integer between first-valid-geoid and last-valid-geoid. In particular, 0 is not a valid geoid (they actualy start at 1)

• Geoids which are close together represent geographic locations which are close together, but the reverse is not true: there are geographic locations which are close together, but their geoids are very different

• The projection method, while not uniform accross the globe, it only has a small distortion and there are no singularities anywhere, including at the poles

• Geoids split the earth surface at different levels: the highest level is level 30, where the earch surface is split into 6 faces. At each lower level, the geoids are split into four, for example at level 29, each of the 6 faces are split into four, producing 24 total geois. The subdivision goes on until level 0 is reached. At level 0, each geoid represents a patch of earth of approximately 0.7 cm2.

1 Converting to and from Latitude/Longitude

geoid->lat-lng will return the latitude/longitude coordinates corresponding to the center of the geoid and this will introduce an error in the conversion from latitude/longitude to geoid and back. For geoids at level 0, emprirical testing showed this to be less than 9.75 millimeters, which is sufficient for the type of applications intended for this libary. Note that this error is not constant accross the globe, and 9.75 millimeters is the maximum error seen.

The bounding box encloses the geoid minimally, but geoids and bounding boxes don’t overlap exactly, so the bounding box will always be bigger than then geoid and there will be other geoids which are inside the bounding box, but not in the geoid.

2 Storing and Retrieving from a SQLite database

Geoids are unsigned 64 bit values, and more than half of them have the highest bit set. SQLite will store numbers as signed 64 bits and convert unsigned 64 bit numbers greater than the maximum signed 64 bit value to floating point numbers, loosing precision. This means that geoids cannot be stored directly into a SQLite database.

These pair of functions subtract 263 from the geoid (or add that value back) to make sure the value is stored correctly. The ordering of the geoids is preserved, so they can still be used to index geograpic information.

3 Generating Test Data

This function is intended to generate geoids for unit tests.

steps specifies the number of geoids ot put on each side of the rectangle, while closed? specifies if the first geoid should be duplicated as the last element in the list, to close the loop.

As with leaf-corners, geoids are placed in counter-clockwise order, but there is no guarantee about the start corner of the loop.

4 API Documentation

See geoid-stride to determine the amount to add to a geoid to obtain the next geoid at the same level.

This can be used to create half-open geoid ranges, for example by leaf-span.

Geoids produced by lat-lng->geoid are at level 0 and you can use enclosing-geoid to obtain a geoid at a higher level.

The returned geoids are not in any particular order.

All geoids which are inside this geoid, regardless of level, are contained in the returned number range, so this range can be used to check if any geoid is inside this one.

The leaf span returned by this function can be used to search for geoids in an SQL query, however, if you do that, make sure you adjust them, as noted in the SQLite section above.

This a convenient function, but if you need to check lots of geoids, this will be slower than obtainging the leaf-span of this-geoid and checking of other geoids are inside the returned range.

Normally, 8 geoids are returned, but only 7 are returned if geoid is in a corner of a face and only 4 geoids if it is a face level geoid.

These functions can be used to get a general idea of the surface covered by a geoid.