Making custom proj4-string with three parameters from old (1975) map

Question

I am looking at an old map (the 1975 layer) and they only have three parameters for their projection:

• Projection: Rectified Skew Orthomorphic (Metric Grid)
• Spheroid: Everest
• Origin: 4.00 N, 102.25 E

I've been trying to make a custom proj4-string in QGIS 3.4 but it keeps returning that it's invalid.

Here's the string:

+proj=omerc +lat_0=4.0 +lon_0=102.25 +ellps=evrst69 +units=m +no_defs

Does anyone know what else I'm missing to make the string a valid projection?

There isn't much else on the map, unless there actually is something on the map that I'm completely missing.

For future reference, what are the things that I need to look out for from old maps such as the one in the link above to try and make the custom proj4-string?

Answer 1

In QGIS 3.18 The projection is given in the picklist for Malaysia

Everest_Modified_1969_RSO_Malaya_Meters

Proj4

+proj=omerc +no_uoff +lat_0=4 +lonc=102.25 +alpha=-36.974209437118 +gamma=-36.869897645844 +k=0.99984 +x_0=804670.24 +y_0=0 +ellps=evrst69 +units=m +no_defs

The second part of the question: when looking for old coordinate systems look in https://www.asprs.org/asprs-publications/grids-and-datums here you'll find the columns by Clifford Mugnier, describing old coordinate systems for a lot of countries. Singapore is https://www.asprs.org/wp-content/uploads/2012/05/01-2006-singapore.pdf

Answer 2

QGIS prefers WKT format, not Proj4. Your definition is missing, at minimum, "Azimuth", "False Easting" and "False Northing". Here's an example of a Custom Variant 2 Hotine Oblique Mercator Projection that I just created in QGIS:

PROJCS["YellowstoneRiver_OMerc_AC", GEOGCS["GCS_NAD_1983_2011", DATUM["D_NAD_1983_2011", SPHEROID["GRS_1980",6378137.0,298.257222101]], PRIMEM["Greenwich",0.0], UNIT["Degree",0.0174532925199433]], PROJECTION["Hotine_Oblique_Mercator_Azimuth_Center"], PARAMETER["latitude_of_center",47.05], PARAMETER["longitude_of_center",-104.65], PARAMETER["azimuth",31.0], PARAMETER["rectified_grid_angle",31.0], PARAMETER["scale_factor",1.000105], PARAMETER["false_easting",656167.979], PARAMETER["false_northing",738188.976], UNIT["foot",0.3048], AXIS["X",EAST], AXIS["Y",NORTH]]