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I would like to digitize a scanned map, see example below.

Example

The projection and datum information is available on the map. I am familiar with georeferencing of a scanned image that contains multiple latitude/longitude coordinates.

Unfortunately, this particular map has only one lat/long coordinate, map's geographic centre, plus distance measurement (i.e., in yards). I am thinking maybe I could convert the distance measurement into degrees, but I don't know how. Is this possible?

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    The map has a grid and coordinate values in meters or yards for both axes. You get excellent ground control points from the crossing points of the grid. – user30184 Sep 28 '18 at 5:54
  • Yes i understand that. However, I do not know how to work with non lat/long coordinates as control points. How do i that? Thanks. – ultrajohn Sep 28 '18 at 5:58
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    You measure the ground control points and feed in their coordinates in the same way than lat/long coordinate. The hard part is to understand the coordinate system of the map and how to tell QGIS in which coordinate system the measured ground control points are. In simple cases it means just giving the correct EPSG code but this CRS seems to be more tricky. Perhaps this may be used as a starting point asprs.org/wp-content/uploads/2018/03/06-17-GD-Philippines.pdf. – user30184 Sep 28 '18 at 6:16
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If you recognize the map area on google, then it is easy to pick-up the lat longs(though they may be cross-checked from known sources) from it and you can proceed with the georeferencing.

Best of luck, sekhar

  • i think i can try this one. – ultrajohn Sep 28 '18 at 6:24
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Details given by Clifford C Mugnier and the map collar:

Projection: Polyconic projection Center of projection: 122° East Map units: us-yards False Easting: 1000000 yards Datum: Luzon (approximate)

I get good soultions with this custom CRS:

+proj=poly +lat_0=0 +lon_0=122 +x_0=914401.8288036579 +y_0=0 +ellps=clrk66 +towgs84=-133,-77,-51,0,0,0,0 +units=us-yd +no_defs

The result is off about 100 meters, due to the "approximate" Luzon datum we do not know. and the scale factor of the projection is unknown too.

  • May I know how you georeferenced the scanned map using the information you provided above in QGIS? – ultrajohn Sep 28 '18 at 12:42
  • Select a grid crossing, and add the imprinted Easting and Northing coordinate to the GCP list. Continue for all grid crossings. For the map CRS, take the custom CRS above. – AndreJ Sep 28 '18 at 13:19

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