HERE Lanes
HERE Lanes Data Specification

# HERE Lanes Coordinate Encoding

Coordinate values for HERE HERE Lanes are relative to the WGS84 (G1674) GCS, and expressed as decimal degrees with 7 digits of precision after the decimal point. In the Protocol Buffer representation, we apply a couple of techniques to minimize their storage requirements.

Rather than using floating point values, we represent latitude and longitudes as 31 and 32-bit signed integers, respectively. We apportion the -180º to 180º degree longitude span over the available 232 range of discrete values. For latitudes in the -90º to 90º range, we use a reduced 231 integer value, as the range is half.

This results in an each sequential integer value being

360° / 232 = 180° / 231 = 0.00000008381903171539306640625 degrees apart

This corresponds to a resolution of around 1 cm at the meridian/equator. The resulting lat/lon integer values are then encoded as Morton Codes, interleaving their bits into a single 64-bit signed integer:

1. First bit is always 0, so all values are positive
2. 2nd bit is leftmost bit of longitude (bit 32)
3. 3rd bit is leftmost bit of latitude (bit 31)
4. Remaining 61 bits alternate between longitude and latitude, ending with bit 1 from longitude as it was one bit longer.

Here is a walk though for a sample location in Sydney, lat/lon -33.86663, 151.20578:

1. Convert the lat and lon values to their integer segment units. (Note: You should always "floor" the resulting number.)

Lat: -33.86663 / (180°/231) = -404044634.09925688888888888888889 = -404044635

Lon: 151.20578 / (360°/232) = 1803955222.4060302222222222222222 = 1803955222

2. Convert the integer lat/long values to their 31 and 32-bit two-complement binary representations:

Lat: -404044635 = 11100111 11101010 11000100 10100101 -> (strip high-bit)

= 1100111111010101100010010100101

Lon: 1803955222 = 01101011 10000110 00101100 00010110 -> (keep high bit)

= 01101011100001100010110000010110

3. Interleave the lon and lat with a leading "0" to produce a 64-bit integer: