[PROJ] Help with impossibly elusive horizontal datum

[Landon Yarrington]

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
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