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I was trying to confirm some GPS coordinates before my last trip to Joshua Tree and found this list which is similar to this list (Appendix IV), but when I try to convert the coordinates to the usual lat/lon, they don't really match up.

It seems that Easting/Northing coordinates can be defined in different ways depending on map projection type or offset origin?

I tried converters like https://www.engineeringtoolbox.com/utm-latitude-longitude-d_1370.html or http://www.zonums.com/online/coords/cotrans.php?module=14 (zone 11?)

For example, Mustang Ranch is listed at 3768017, 575615 which converts to 34.05013, -116.18072:

Map of Mustang Ranch

but is actually closer to 34.05194, -116.18145:

Map of Mustang Ranch

Headstone Rock - South Face is listed at 3760548, 578384 which converts to 33.98257, -116.15139:

Map of Headstone Rock - South Face

but the actual location is more like 33.98439, -116.15240:

Map of Headstone Rock - South Face

Do these seem like conversion errors or measurement errors?

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    It does look like a datum error, the correct locations are about the same magnitude and direction, it could also be the locations were determined from a cell phone - their GPS units are really horrible outside of suburban areas (see gps.gov/systems/gps/performance/accuracy and researchgate.net/publication/…). It could also be that Google Maps that far away from anything of significance the image may not be orthorectified very well. Feb 14 '20 at 3:35
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    Most likely, the coordinates are not in WGS84 datum, but the source does not say what datum they are in. Feb 14 '20 at 3:42
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The coordinates are based on the NAD27 datum, converted to UTM zone 11N.

You can use the TAGIS Converter to see them and get the WGS 84 geographic coordinates.

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  • That works! How did you figure it out? Maybe when they were making GPS measurements they were trying to match the coordinates to (old?) paper maps?
    – endolith
    Feb 14 '20 at 20:39
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    I think that those coordinates were measured a long time ago, and then they were copied from document to document without clarifying based on which datum the original measurements were made. Before the era of satellites, it was not so easy to measure geographic coordinates, then there were fixed points referenced in some Cartesian systems and the surveys were linked to them. The fact that the documentation had UTM coordinates instead of geographical ones hinted at such a measurement. Feb 14 '20 at 21:18
  • Would you happen to know of a command line to convert a file of coordinates from one to the other? I can arrange them in whatever format is required, but I don't know all the different conversions and aliases for them. "For example, EPSG:4326 and EPSG:4267 are reference codes for WGS84 and NAD27", etc.
    – endolith
    Feb 15 '20 at 18:22
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    Yes, they are. EPSG:26711 is NAD27/UTM zone 11N, the axis order is Easting, Northing. EPSG:4326 is WGS84/Geographic coordinates, the axis order is Latitude,Longitude. Feb 15 '20 at 20:17
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    Thanks! It worked: drive.google.com/open?id=1-l74mnV837FMbQ_7th14kTkIMbm-gPyB
    – endolith
    Feb 15 '20 at 22:00

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