Converting elevation grid to .png

Dean MorrowDean Morrow Global Mapper UserPosts: 12
edited February 2013 in Raster Data
Hi

I am converting some elevation and vector data to .png for a club website. These images are then being used by another guy who is importing it into the website. He is using pixels to geo reference the images. They line up well with existing data I have and can be brought into google earth without problem but when they are brought into the website things don't line up. The latitudes look close-ish but the longitudinal arcs seem to be causing some issues. Is there something I can do when I export to keep things in order or is it up to the website guy to make adjustments?

Thank you

Dean

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2013
    Dean,

    What projection are you exporting to? IF you export to KMZ for Google Earth it will also be as lat/lon/WGS84. Other formats support whatever projection is on the Projection tab of the Configuration dialog and will have different shapes.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • Dean MorrowDean Morrow Global Mapper User Posts: 12
    edited February 2013
    Mike, I'm exporting to geographic lat long WGS84
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2013
    If exporting in Geographic/lat/lon the longitude lines should be straight up and down in the PNG images. Is that not what you are seeing?

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • Dean MorrowDean Morrow Global Mapper User Posts: 12
    edited February 2013
    Yes the lines are straight up and down. The problem is that as we spread further east and west of the center of the image points and lines created after my export get further out of correct position. Down the center of the image things look pretty close and stuff is a little out of position in the north and south directions but not nearly as bad. The area is about 270 nm top to bottom and 290 nm left to right. By the time we get to the far west or east of the image the points are about 12 miles off. This may be a problem with how the webpage is set up but I wanted check to see if there was something I was missing. I should add that when I export I use the "calculate spacing in other increments" to use meters instead of arc degrees so I can get a feel for the size of the image. Sometimes the dimensions are not square but when I change them I make them square for example the spacing might be 42 by 50 meters and I change it to 100 by 100 meters. I haven't had to do this every time so I don't think it is the main issue but it may be contributing to it.

    Thanks
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2013
    How are you determining that the position is off? There won't be a constant scale over your image in Geographic and the actual ground size of the X and Y pixels will actually be different depending on where you are on the earth as you can't perectly model the surface of the earth in a 2D Cartesian coordinate system. You will get less scale difference if you switch to a projected system like UTM, which should be fine for an area that size. Then normal linear measurements will be much closer. For any accuracy over large areas you would need to do real geodetic calculations though and not simple Cartesian ones.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • Dean MorrowDean Morrow Global Mapper User Posts: 12
    edited February 2013
    Mike, once the image is brought into the webpage using the corner points as reference like this xx.xx N .xx.xx W then you can add points or other image overlays etc. So I created some points, graticules and contours and exported from GM and they of course match with the existing images but if anyone creates any vector data directly on the webpage after that they don't land in the same spots. I'll check to see if we can change over to UTM with out too much trouble. Do you think that would solve the issue?

    Doing any geodetic calculations is beyond my experience, I am hobbyist at best. Maybe the other guys are able to resolve it on their end.

    Thanks much for the help!

    Dean
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2013
    Dean,

    If you create new data directly on the image you have to make sure to calculate the X and Y coordinates in degrees since that is the native "projection" of the data. The X and Y in degrees longitude and latitude should scale linearly. You can't use ground units like meters as those are linear units and degrees are angular units, so there is not linear relationship between them.

    If you use UTM you will get a much less distorted image and your calculations will be directly in meters. However UTM will become highly distorted if you get very far (i.e. 1,000 miles or so) from the UTM zone center. For an area of your size it should be fine though.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • Dean MorrowDean Morrow Global Mapper User Posts: 12
    edited February 2013
    Okay Mike, I'll keep at it.

    Thanks again.

    Dean
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