[PROJ] Help with impossibly elusive horizontal datum

[Jose Gonçalves]

Hi
I looked at the map grid GCPs you wrote in the gdal_translate command. The
first one is: -gcp 730.736 6110.949 789000 2158000, but for that image
position the UTM 18N coordinates are (789000,2159000). May that explain
part of the error?
Regards
José



Em ter., 2 de jul. de 2024 às 04:20, Landon Yarrington via PROJ <
proj at lists.osgeo.org> escreveu:

> Hello!
> I am trying to georeference an US Army map of northern Haiti made from
> aerial photographs taken in 1942 and 1944 using `gdal_translate` and
> `gdalwarp`. I need a sanity check because this is impossible, and so I'm
> asking for help. Here is the map from Lib. of Congress (
> https://www.loc.gov/resource/g4940m.gct00206/?sp=160&r=0.365,0.623,0.281,0.173,0
> ).
>
> The map info states,
> ```
> Horizontal Datum is based on the following astronomic values:
> Fort Islet Lighthouse Lat. 18°33'31.33", Long. 72°20'59.03"
> Santo Domingo Lighthouse Lat. 18°27'53.64", Long. 69°52'52.74"
> Cape Dame Marie Astro Lat. 18°36'47", Long. 74°25'53"
> Extensions of control from Astronomic Positions have been made by
> multiplex triangulation.
>
> One thousand meter Universal Transverse Mercator grid, Clarke 1866
> spheroid, zone 18.
> ```
> I asked something similar in 2017 (
> https://gis.stackexchange.com/questions/255830/which-epgs-code-is-the-actual-crs-in-this-geopdf),
> but the advice doesn't seem directly applicable (even after adding the
> missing `towgs84` parameter). The two mapes portray the same geographic
> content, but the grids are pretty different with about ~1,000 meter offset.
> I guess this is because the map was projected with a custom (??) datum, and
> not NAD27?
>
> The very best accuracy to WGS84 I've been able to get with the 1st edition
> LOC map is ~1,000 meters offset. I think there must be some transformation
> involved, but it isn't clear to me at what point in the process. Should the
> Eastings and Northings in the map be transformed and use those transformed
> values as GCPs? Or should the GCPs use the grid values and some transform
> happen later? Or should is this just a really elusive horizontal datum...
>
> I've tried everything I can think of. Here is a starter using lat long GCP
> points on the LOC map image.
> ```
> gdal_translate -gcp 627.457814 517.220980 -72.25 19.666667 -gcp 8893.0877
> 517.331604 -72 19.666667 -gcp 8888.9205 6312.2196 -72 19.5 -gcp 623.864944
> 6314.5746 -72.25 19.5 -of GTiff "GRN.tif" "GRN_gcps_latlong.tif"`
>
> gdalwarp -order 1 -r bilinear -s_srs '+proj=longlat +ellps=clrk66' -t_srs
> '+proj=utm +zone=18 +ellps=clrk66 +towgs84=-13,-95,-197,0,0,0,0 +units=m
> +no_defs' GRN_gcps_latlong.tif GRN_projectedtif
> ```
> If you prefer, here it is with 10+ GCPs using the map's grid.
> ```
> gdal_translate -of GTiff -gcp 730.736 6110.949 789000 2158000 -gcp 736.387
> 5795.335 789000 2159000 -gcp 741.696 5481.115 789000 2160000 -gcp 747.267
> 5165.909 789000 2161000 -gcp 821.443 766.88 789000 2175000 -gcp 816.24
> 1078.971 789000 2174000 -gcp 811.211 1394.578 789000 2173000 -gcp 6501.597
> 551.782 807000 2176000 -gcp 6185.673 547.045 806000 2176000 -gcp 4288.393
> 829.835 800000 2175000 -gcp 3975.028 824.02 799000 2175000 -gcp 3658.991
> 818.975 798000 2175000 -gcp 3344.584 813.972 797000 2175000 -gcp 3027.108
> 809.96 796000 2175000 -gcp 2711.015 805.008 795000 2175000 -gcp 2397.12
> 800.185 794000 2175000 -gcp 4283.082 1142.947 800000 2174000 -gcp 3969.927
> 1137.041 799000 2174000 -gcp 3653.994 1131.957 798000 2174000 -gcp 3339.13
> 1126.642 797000 2174000 -gcp 3021.555 1121.964 796000 2174000 -gcp 2706.024
> 1116.956 795000 2174000 -gcp 4272.358 1768.053 800000 2172000 -gcp 3959.532
> 1762.746 799000 2172000 GRN.tif GRN_gcps.tif
> ```
> I am indebted for any help to solve this!
>
>
> --
> Landon Yarrington, Ph.D.
> Colorado State University
> Department of Anthropology and Geography
> Andrew G. Clark Bldg, 301 University Ave #219
> Fort Collins, CO  80521
> USA
> _______________________________________________
> PROJ mailing list
> PROJ at lists.osgeo.org
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>
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