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I'm trying to georeference this part of an old map of Indochina.

Map to digitize.

The full extent gives more information:

full map.

To do so, I used QGIS and tried to infere the datum, I tried "Unknown datum based on Everest", but it gave me a big difference that you can see in this screenshot:

qgis.

I did it with the three geodesic intersections available in the map. To understand the problem, I looked at Luang Prabang (the city close to the intersection of geodesics on the top of the map) coordinates and found 19.53 N and 102.80 W on geonames.org. On my map, it is more about 100 W, that is a big difference.

Thus my questions:

  • given that it is an old map, does the inaccuracy result from the mappers who did the map (hats off for there impressive work btw !) or is it that I do not have the right method (datum, choice of points, ...) ?
  • how can I georeference this map ? (My goal is to digitize the borders).
PolyGeo
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syldor
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    Have you looked at these similar posts?: http://gis.stackexchange.com/q/43682/8104; http://gis.stackexchange.com/questions/103607/how-to-exactly-georeference-in-qgis?lq=1 – Aaron Oct 13 '14 at 12:40

2 Answers2

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The Map has mostly remarks in French, so I would suggest to use a French coordinate system, and not Everest (who was British).

The French used coordinate systems with a prime meridian based at Paris, so EPSG:4821 Voirol 1879 (Paris) is a good start-off. With that, the 100° meridian is hitting the Malaysian coast near Kota Baru, as in your map:

enter image description here

You can use the imprinted grid coordinates to georeference your map.

The other way is to use as many reference points as possible. Bing roads and Openstreetmap have many of the town names of the old map. With 47 control points, EPSG:3857 (as Openlayers plugin requires), Thin Plate Spline and next Neighbour interpolation I get this result:

enter image description here

As noted in another answer, a good flat copy is essential for georeferencing.

AndreJ
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  • Try to find as many known points on the outside border of the map. Also going in one direction around the map. I generally use clockwise. – Brad Nesom Oct 13 '14 at 12:31
  • I feel like this is the solution ! However, when I reference with Voirol when loading my raster, it outputs the same referencing as everest, it should not be the case ! I output both on ESPG:4326 with a linear projection. I must be missing something ... – syldor Oct 13 '14 at 16:31
  • I guess you have not yet collected any ground control points. – AndreJ Oct 14 '14 at 03:29
  • I just saw your edit now, it looks very promising. To be sure I understand, I need to put the openstreetmap layer in my canvas, load my map with EPSG: 3857 then select fifty points on the old map and select the equivalent fifty on the openstreetmap canvas ? – syldor Oct 15 '14 at 04:22
  • I used the cities I could match with the bing roads layer and the result is great. – syldor Oct 15 '14 at 04:55
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To begin with I would recommend you read this tutorial on georeferencing in QGIS http://docs.qgis.org/2.2/en/docs/training_manual/forestry/map_georeferencing.html Yes you can georeference this map. I would recommend you use a good quality flat scan of the map if possible. Avoid a crease running through the image. Accurate georeferencing depends on the quality of the scan, the number of coordinate points, and the transformation settings. Look at these questions also. How to exactly georeference in QGIS How can I improve my Georeferencing results?

Community
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Richard
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