Georeferencing old map in QGIS?

Question

I have an old map over a part of Laos and wondering how do I georeference the map with the help of the Grid lines and other Longitude and Latitude information on the scanned map? I have searched for information on the internet and tried many times. My scanned map wont overly correctly on the base map. Any base map would be fine. As long as I can check the result.

Do I add X/East and Y/North values from the grid line crossings as marked and written in some parts of the map? How do I begin georeferencing this map? Do I have to add zeros after the two digits? For example if the Easting is 24 (its in KM right? Do I add like three/four zeros to convert it to meters?

I georeferenced the map with the option "From Map Canvas" in the georeferencer tool and I succeeded but I want to know the coordinate of a point on my old scanned map and write them as X/East and Y/North to georeference the map.

I hope I am not babbling and someone understands what I mean!

We can set the CRS to WGS84, EPSG:4326.

Answer 1

It looks like the map is aligned to lat-long and the grid system is skew to that slightly. The map corners have lat-long markers and seem to confirm this. So in the QGIS georeferences, tag the four corners and give them the coordinates in the corners - you should get this:

The map is from 100 to 101 degrees E and 20.67 to 21.33 N. Once those points are set, doing a linear transformation makes for a pretty good reference with OpenStreetMap base maps:

Answer 2

The map text reveals the use of the Vientiane datum, Krassovsky ellipsoid, and Gauss projection.

So my first try is the same as Spacedman uses, but using all edge degree imprints, and the one in the middle; using EPSG:4676.

To get the projection right, I digitized the imprinted square grid for every full 20 km, adding the prefix 23 for Y and 176 for X. Setting the project CRS to the locally valid UTM 47N, the latitudes appear exactly horizontal. So the Gauss-Kruger transverse mercator projection seems to use a meridian of 99 degrees.

Measuring the distance from that meridian to the map, the false Easting seems to be 17500000 meters.

So the projection can be built as:

+proj=tmerc +ellps=krass +lat_0=0 +lon_0=99 +x_0=17500000 +y_0=0 +k=1 +units=m +no_defs

This resembles a lot to the 6-degree Gauss-Kruger zone 17 that has been used in Russia and China.

For parts of Laos East of 102°, they might have used the GK zone 18, which is used in Vietnam as well (EPSG:2044).

Here you see the custom CRS 20km-grid in blue underlying the digitized gridlines in red:

Answer 3

Your map has clearly defined coordinates on its corners. Then the process should be straightforward. You just need 4 georeference points (which is minimum for georeference) and place them exactly in the map corners. Then in the transformation options select proper CRS, which I guess can be WGS 84 and method (preferably projective). You can also use Raster -> Extraction -> Clip raster by extent

You can read this article, where the method was shown how to do it really quickly.

https://guides.lib.utexas.edu/georeference-raster-data/qgis-georeference-by-map-corners