ASTER GDEM elevation data with Google Maps Satelite Images

richardsrichards Global Mapper UserPosts: 5
edited June 2014 in Projection Questions
Hi there,

i am very new to GIS and Global Mapper.
I just would like to combine and perfectly align elevation data from ASTER GDEM V2 (this should be in UTM / WGS84) with satelite imagery from Google maps (that should be in (pseudo) Mercator projetions based on a sphere.

I downloaded 1 tile of Aster gdem (N54 E013 - island of Ruegen, Germany) and the appropriate part with ultimate maps downloader from Google Maps in Zoomlevel 13 or 14.

What is the best way to align both maps and get a perfect fit all over the map?

Do I have to reproject both part to another projetion / datum or would I just reproject one of them?
I tried to get the googlemap image to UTM / WGS84 but i never get a perfect fit.
I tried it the other way round but no perfect fit.

Do i misinterpret the initial native projection of one of the two?
Is there something wrong with the process?

I start with importing the Aster DEM and assume the whole project is set to the native projection as included in the Geotiff of the DEM, right? Then it should be enough just to reproject the satelite image?

Thanks for any hints!:)

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited June 2014
    The ASTER GDEM data should load in with its proper projection to start with, then when you load an image from Google Maps use Mercator with the 'Google Maps' datum to get the special system used by Google Maps and many other online sources.

    I would also try using the File->Download Online Imagery/Data menu command to pull down some other imagery sources.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • richardsrichards Global Mapper User Posts: 5
    edited June 2014
    Thanks Mike, for your quick answer.

    I think i do something wrong in my workflow due to misunderstanding of GM functions.

    Thats what I do:

    1. Load Aster GDEM - imports fine with correct projection (geographic lat/long, WGS84, ARC Degrees)
    2. Load satelite image of the exact same geographic position from ultimate maps downloader (combined image) -
    b. i check manualy rectify image
    c. i add the four corner points which represent just 1 tile in lat/long, i.e.: N55-E13, N55-E14, N54-E13, N54-E14
    d. I select Projection as Mercator, Sphere (maybe the datum is wrong ? - but no difference when I choose WGS84 e.g.)
    e. I choose not to reproject the points but to interpret them in this projection

    I get 2 layers that fit together on the corners but on the islands outline the data doesnt match exactly, the layers are not congruent.
    What is wrong in my workflow?

    I am not sure if i really understand what the projection tab in the rectifying window means. As i understand it, its not used for reprojecting the whole project but to interpret the rectifing data points in that indivudual projection for this layer, right?

    Thank you!
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited June 2014
    If you are rectifying images directly from Google Maps, the projection is Mercator and the datum 'Google Maps'. This is a special hybrid of WGS84 and a simple sphere. You have to use the Google Maps datum to get the correct interpretation, assuming that is how the Ultimate Map Download software pulls the data.

    But if you email geohelp@bluemarblegeo.com we can provide information on how to directly pull that data in to GM as an online source so you don't have to rectify the data at all.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • richardsrichards Global Mapper User Posts: 5
    edited June 2014
    Thanks a lot Mike!

    I solved the problem :-)
    1. problem was I should have choosen "reproject GCP-point" to new projection" (not just interpret...)
    2. problem was Ultimate maps downloader didnt stick to the exact coordinates you can enter there, but gets you a slightly bigger
    map than really choosen lat/long.

    Now I got an exact match!

    Thanks for the hint with directly pull data - will try this too :-)
    The only thing is my webconnection is rather slow and i try to download and combine all the maps before i start my work with mapping.
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