Raster maps and datum

arnaldo.maurizioarnaldo.maurizio Global Mapper UserPosts: 7
edited August 2012 in Technical Support
Hi all. I'm new to this forum, looking for help in the use of raster maps on Globalmapper. I searched the web for detailed maps of Morocco. I could ind a good amount for free at this site http://www.lib.utexas.edu/maps. There are severals. I am interested in 1:50.000 even if dated. The orographic contours definitely are not changed and the existing roads probably traces the old ones or are very closed. http://www.lib.utexas.edu/maps/ams/morocco_50k/ for many maps are available projection data and its characteristics, this is an example (http://www.lib.utexas.edu/maps/ams / morocco_50k/txu-oclc-6589697-29-11-4a.jpg) then I can use a UTM. There is no mark about to the geodetic datum used for the generation of the map, in particular I would be interested in the transformation parameters into WGS84. If someone has already faced similar problem and solved it, I'd be grateful.
Arnaldo

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2012
    Arnaldo,

    I looked at once of the maps and the bottom seems to indicate that the ellipsoid of the maps is Clarke 1880, so you could create a custom datum based on the Clarke 1880 ellipsoid with all zeros for the transform parameters and use that. That should be good enough give the accuracy of the maps.

    You might also try some of the online sources, like World Topo Maps or World Street Maps, available on the File->Download Online Imagery/Data menu command to see if those work as they are already positioned for you.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • arnaldo.maurizioarnaldo.maurizio Global Mapper User Posts: 7
    edited August 2012
    Thanks a lot for your fast answer. I've tryed this customized solution " Clarke 1880, 34N and Greenwich meridian, false coordinates at 1000000East and 50000North meters, 0.999080 scale factor" for the projection. What I miss is the Datum from what they have used to restore WGS84.
    With World Street Maps or other sources, it really is not positioned. Thanks
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2012
    How far off is that solution? At 1:50K scale for maps that old I would expect just getting the ellipsoid right would probably get you to within the error of the maps. If you pull in World Imagery can you use that to position the maps?

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • arnaldo.maurizioarnaldo.maurizio Global Mapper User Posts: 7
    edited August 2012
    Thanks, this is really not a "tollerance" problem. The map is far away its natural position. Too difficult without any city reference. Quite impossible to position a 1:50000 scale only with orographic contours. I've tryed with GoogleEarth, some cities can be located.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2012
    It doesn't sound like a datum is the issue then is you are that far off. Can you point me at a particular one that you are rectifying and provide a workspace with your control points and projection then I can see what is going wrong. What projection are you choosing to go with the parameters that you stated?

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • arnaldo.maurizioarnaldo.maurizio Global Mapper User Posts: 7
    edited August 2012
    Hi, thanks againn. This is a sample of map: http://www.lib.utexas.edu/maps/ams/southern_morocco/txu-oclc-6596235-sheet-43.jpg we will work with this 1:125.000.
    Tools / Coordinate Convertor / Select Input Coordinate System from Globalmapper. Then from "Projection" scroll down menu, I selected Lambert Conformal Conic, Datum Merchich(Morocco) this probably is the closest Datum. Planar Units into Meters, then I entered the attribute parameters for Northwest Africa Zone Grid:
    Scale Factor: 0.999080; First Standard Parallel: 30,00; Second standard Parallel:30.30 (bottom left of the sheet) ; Central Meridian: 0; Origin Latitude 34, False East(m) 1000000; False Origin North(m): 500000.
    On the output Coordinate Sytem, Trasverse Mercator, Datum WGS84, Meters.( 90000E, 150000N; -9192910.79676E, 3414413.55038N) ( 130000E, 140000N; -872531.33469E, 3404261.52534N) ( 110000E, 100000N; -892754.82392E, 3363892.58588N). Honestly the minus is suspicious.
    With this three points I calibrated the sheet with no success. If you would provide me your direct e-mail thank I could send you some pictures. My one is arnaldo.maurizio@alice.it Thanks
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2012
    I took a look at this and was able to get it properly positioned. I just loaded the JPG and when prompted selected YES to rectify it. I then set the projection up based on the Northwest AFrica Zone Grid using Merchich as the datum. The dark grid lines in the top 80% of the image are on that grid. I then added 3 control points using those grid lines then pressed ok. I loaded in World Street Maps and verified that it lined up, which it did quite nicely with some features I could find.

    I have attached a workspace showing my rectification. Just put this in the same folder as the JPG file and load it and the data should come in.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • arnaldo.maurizioarnaldo.maurizio Global Mapper User Posts: 7
    edited August 2012
    Did you set up the data for the Northwest Africa zone grid manually as per data: Scale Factor: 0.999080; First Standard Parallel: 30,00; Second standard Parallel:30.30 (bottom left of the sheet) ; Central Meridian: 0; Origin Latitude 34, False East(m) 1000000; False Origin North(m): 500000 and Merchich as datum??
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited August 2012
    I'm not sure where you see that, I'm using the definition in the bottom center. It is a Lambert Conformal Conic with a scale factor, so the first and second standard parallels are set to the same as the origin latitude, all at 34, then the rest as you said. Another way to do this is simply use the 4 corner coordinates which have lat/lon associated with them. Just set up any projection on the rectification dialog that is good and set the datum to Merchich, then enter the lat/lon values at each corner. Make sure to use -9 and -9.5 for the longitudes since they are in the Western hemisphere.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • arnaldo.maurizioarnaldo.maurizio Global Mapper User Posts: 7
    edited August 2012
    Ok, thanks. I am going to set up again everithing. Meanwhile I wrote to the "Department of Geography University of Texas at Austin". They suggested me to have a look at this document: http://www.asprs.org/a/resources/grids/06-99-morocco.pdf this is dedicated to Morocco. I think this forum coud thake advantage from the documentation at the American Society for Photogrammetry and Remote Sensing http://www.asprs.org/a/resources/grids/ Really thanks for your help.

    Arnaldo
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