3

I have a 1906 scanned map that I am trying to georeference in QGIS for Mac. The map has French and English lat-long, indicating them as either "Meridiano de Greenwich" or "Meridiano de Paris." I would like to know what the closest approximation to "Meridiano de Greenwich" for choosing the CRS might be.

  • This is a question of datum. The map says it is based on English and US naval maps. At the time the US was working with two datums (global models), the Hayford ellipsoid and Clark 1866 ellipsoid. The latter was likely the basis for maps within and around the North American Continent (and ultimately became the basis for the NAD 1927 datum then under development). A good default choice, then, would be the Clark 1866 datum. (The US Navy was supported by the Coast and Geodetic Survey; a brief history is at lib.noaa.gov/noaainfo/heritage/coastandgeodeticsurvey/…) – whuber Apr 3 '14 at 19:45
  • True, but the Americans began their surveys on the island in the 1920s. See asprs.org/a/resources/grids/12-2005-dominica.pdf where the map in question is also mentioned. – AndreJ Apr 4 '14 at 5:26
2

My Spanish (Portuguese?) is rusty but I'm going to guess that it refers to the Greenwich Meridian, so your map is probably in some form of Lat/Long with 0 degrees at Greenwich so try EPSG:4326 first.

To really determine the projection we'd need more details than just the prime meridian.

  • Trying this datum, I find that for whatever transformation option I select, there is significant diagonal skew in the SE-NW direction. I am trying only 4 GCP at the intersections of lat-long from the scanned map. This makes me think it's the wrong CRS??? – grad student Apr 3 '14 at 16:38
  • BTW, I am selecting EPSG:4326 when I first open the raster and when projecting the map onto my shapefile. – grad student Apr 3 '14 at 16:43
  • can you post an image of the map? – Ian Turton Apr 3 '14 at 16:59
  • I can't post an image because it's too big, and a screenshot is more hassle than just posting the link from the Lib of Cong from which I obtained the map. 1906_hispaniola – grad student Apr 3 '14 at 17:05
  • I should say that I've tried making GCP directly from the map canvas-to-shapefile option with mixed results. This may be because I enter too many GCP or do not space them out far enough though... – grad student Apr 3 '14 at 17:18
1

Second try, with EPSG:4326 and the following GCP:

mapX,mapY,pixelX,pixelY,enable
-74,20.1666666666700003,2053.94253295474391052,-2419.17835250553116566,1
-73,20,5225.22388495872564818,-2983.42925651106543228,1
-72,20,8392.67980710504343733,-2983.42925651106588703,1
-71,20,11579.26287853968642594,-2991.08011622639514826,1
-70,20,14754.36966040133665956,-2991.08011622639514826,1
-69,20.1666666666700003,17933.30187212064629421,-2421.09106743436450415,1
-71,18,11590.73916811267554294,-9723.83666571615322027,1

works reasonably well, compared to Natural Earths world boundaries:

enter image description here

You can improve the result by cutting the map along the foldings. But take in mind that surveying on Haiti was rather basic those days.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.