# Georeferencing map with known Central Lat-long and distances?

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.

• Coordinates of lower left corner are 2 out of 6 lines in world file. 2 others can be obtained by dividing height,width by number of rows, columns (pixel s) of the image. Rotations set to zero Jan 9, 2017 at 8:36
• @FelixIP good point, I've expanded on a few things in an edit. I didn't really want to duplicate other answers that form part of this one here, so the OP may have to go look for how to create a world file properly. Jan 9, 2017 at 8:43

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.