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I have the scanned copy of an Old Map which was created using a Plane Table survey, and based on my reading of literature around it, is supposed to be quite Accurate.
The central Lat-long is known, as is the fact that this map represents an Area Of 1000 feet(North-South) by 1400 feet (East-West)
How can I Georeference this Map using just these facts?
I have access to QGIS & GDAL.
Here's what I'd do:
convert that central lat-long to a locally planar coordinate system (the nearest UTM zone should do). That gives you a coordinate (offset from the UTM origin) in metres. You can do this by creating a single point at that coordinate in QGIS and then seeing where it goes when projected to your UTM zone.
convert 1000ft and 1400ft to metres using a calculator or online unit converter or something. Google says 1000ft is 304.8m and 1400ft is 426.72m so that's that done for you...
then you can work out the coordinates of the corners of your map in UTM coordinates by adding/subtracting half of 1000ft and 1400ft to the centre coordinate. That gives you enough information to create a World File to go with your map image file, which is the way to register an image file into real-world coordinates. Note that a world file doesn't hold the coordinate reference system, so you need to assign it to the layer when you read it into QGIS. You can then probably save it as a GeoTIFF with the CRS and location data.
Your source is a small area compared to the surface of the globe so as long as it was surveyed as a flat plane and isn't projected in some way then this should be sufficient.
You can make use of the Georeferencer in QGIS.
Before, you have to create a custom transverse mercator projection on the center point:
+proj=tmerc +lat_0=... +lon_0=... +x_0=0 +y_0=0 +datum=WGS84 +units=ft +no_defs
Then load the image into the referencer, click on the upper left corner of the image, and enter the coordinates manually as:
X: -700 Y: 500
continue for the other corners, exchanging the + and - as necessary. In the Georeferencer settings, set the CRS to the custom CRS, and start the georeferencing.
Alternatively, you might try Indian feet: +units=ind-ft and the Everest datum.
