In the png map identify 4 points that can be used easily to georeference
Note down the X and Y coordinates for each identified point
Note if the units are in meters or in feet. See the example below
Point
X
Y
A
6750
-115750
B
9000
-115750
C
8750
-117250
D
9000
-117250
The rater's Y coordinates in this example are negative because the location is South of the equator.
The rater's X coordinates in this example are positive because they are East of the Greenwich Meridian.
2. Converting coordinates
Use the following website to convert
coordinates from cassini to either UTM (WGS84), GCS (WGS84) or Arc 1960. Shambamap used WGS84 4326
However this website is only fit for land parcels in
Nairobi, Kiambu and part of Kajiado and Machakos (See here)
See the screenshot attached below
Be keen to ensure that the coordinate units (the current map is in meters)
are correctly selected before you start the conversion process
3. Georeferencing process in QGIS
In QGIS click on Layer tab and select Georeferencer.
Upload the map in png format that you want to georeference.
Create 4 georeferencing points from the earlier identified points and enter the
converted coordinates. Ensure that you select the correct coordinate system in line
with the converted coordinates
Run the georeferencing process by clicking on the run (play) button. Save the output tiff (image) to
your desired location and ensure the output coordinate system (WGS84 4326) is correct
The georeferenced image is automatically added to QGIS
Verify by overlaying on Google Satellite to see if the image was correctly georeferenced
The following video shows all the steps followed in QGIS to georeference the map