How to convert here_2d_coordinate_diffs or cm_from_WGS84_ellipsoid_diffs into real coordinates or altitude values

Symptoms

• You see here_2d_coordinate_diffs and cm_from_WGS84_ellipsoid_diffs in HERE HD Lanes data, but you cannot tell how to convert them into usable latitude/longitude or altitude values.
  
• Example raw values may look like "here_2d_coordinate_diffs": ["10247998343","461447","106248", ... ], but the numbers do not match readable coordinates by themselves.
  
• Customers also often do not know which reference value to use, especially for tile_center_here_3d_coordinate and its "here_2d_coordinate" field.

Quick Answer

These diffs are not real coordinates by themselves; you must reconstruct each value by applying the XOR operator (^) to the previous real value and the current diff. For here_2d_coordinate_diffs, use the tile center here_2d_coordinate as the initial reference value, and for cm_from_WGS84_ellipsoid_diffs, use 0 as the initial reference value.

Step-by-Step Resolution

1. Locate the tile center reference value in the same HERE HD Lanes file. The article example shows: "tile_center_here_3d_coordinate": { "here_2d_coordinate": "581931217788600320" }.
2. To reconstruct here_2d_coordinate_diffs, use that tile center here_2d_coordinate as the reference value for the first calculation.
3. Apply XOR (^) between the reference value and the first diff. In the example, 581931217788600320 ^ 10247998343 = 581931228036598663, which gives the first real 2D coordinate value.
4. For the second diff and every diff after that, use the previously reconstructed real value as the new reference. In the example, 581931228036598663 ^ 461447 = 581931228036530432.
5. Repeat this logic for the full list to reconstruct all real 2D coordinate values. The example article shows the resulting sequence as [581931228036598663, 581931228036530432, 581931228036505096, 581931228036866582, 581931228036900868, 581931228037003571, 581931228037097472, 581931228038701626, 581931228038804646].
   
6. To reconstruct cm_from_WGS84_ellipsoid_diffs, use 0 as the initial reference value. The first altitude example is 0 ^ 74456 = 74456.
   7. For the next altitude value, use the previous reconstructed altitude as the new reference. The example shows 74456 ^ 13 = 74453.
   
7. Continue the same XOR process through the remaining altitude diffs to reconstruct the full altitude list in centimeters. The example article gives [74456, 74453, 74455, 74462, 74475, 74494, 74520, 74552, 74592].
   
8. The article states that the XOR operator is ^ in programming languages such as C, Java, and Python, so you can implement the reconstruction logic directly in code.
   
   

Verification

• Confirm that the first reconstructed 2D coordinate matches the expected example value 581931228036598663 when using the tile center reference and the first diff.
• Confirm that the second reconstructed 2D coordinate matches 581931228036530432 when using the first reconstructed value and the second diff.
• Confirm that the altitude sequence begins with 74456 and then 74453 when reconstructed from 0 and the first two altitude diffs.
• A common mistake is using the tile center value for every 2D diff instead of only for the first one; after that, each new calculation must use the previous reconstructed value.
• Another common mistake is using the tile center as the initial reference for altitude; the article explicitly says altitude reconstruction starts from 0.