Projection Not Correct

paulp575paulp575 Global Mapper UserPosts: 59Trusted User
edited January 2011 in Technical Support
I'm trying to load a shapefile with the information provided by the source web site.

Problem is, when I follow their projection information, the displayed overlay is way out of place.

Here's the website I'm getting the information from:

http://data.geocomm.com/readme/usgs/grids.html

When the 24k overlay is loaded, it asks me for the projection information.

I don't find the indicated "Lambert Azimuthal Projection". The only one I see is "Lambert Azimuthal Equal-Area" which I'm sure is not the same! When I use that, the northern border of the US (near the NE corner of Washington state) is indicated to be around 5 degrees north! Last time I checked, we were not considered a near-equatorial state!

What am I doing wrong?

Thanks,

Paul

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2010
    Paul,

    That is the correct projection, are you entering the projection parameters as specified, namely -100 for the Central Meridian and 45.0 for the Origin Latitude?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulp575paulp575 Global Mapper User Posts: 59Trusted User
    edited October 2010
    That is the correct projection, are you entering the projection parameters as specified, namely -100 for the Central Meridian and 45.0 for the Origin Latitude?

    Thanks, that worked!

    So how does one know what to do when an overlay does not have projection information?

    Thanks,
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited October 2010
    You need to know the complete projection definition to use for the layer if that isn't already provided in some sort of standard metadata file (like a PRJ file). The web site that you pointed at with the projection definition listed all of the parameters for the projection.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulp575paulp575 Global Mapper User Posts: 59Trusted User
    edited November 2010
    You need to know the complete projection definition to use for the layer if that isn't already provided in some sort of standard metadata file (like a PRJ file). The web site that you pointed at with the projection definition listed all of the parameters for the projection.

    Let me know if I can be of further assistance.

    While that helps, what if you don't have a .PRJ file?

    For example, I am loading USGS DRGEs that have only the .TIF, .TFW. and .FGD files.
    These came from GeoCommunity.com.

    When I load the quad girds (mentioned above) and insert the correct information in the projection dialog box, and then look at what's displayed, something is out of line.

    The quad grids and DRGEs are not lining up; they are offset by half the width of one 7.5 minute quad, i.e., about 3.75 degrees to the east/west(?) and a little out of alignment north/south.

    I first loaded the DRGEs and didn't specify a projection.
    Then loaded the quad grids and had to input the projection information.

    What am I doing wrong (again!)?

    THanks,
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2010
    There is likely something wrong in your projection specification, most likely the datum that you are selecting for a shift of the size that you are indicating. If the DRGEs are coming in without prompting you for a projection then Global Mapper is able to automatically find that information so those are most likely correct. It is the data that you are being prompted for a projection where the problem is likely occurring. Does the page where you are downloading that information say anything about the projection and datum being used? Typically the data provider will indicate the correct projection/datum somewhere.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulp575paulp575 Global Mapper User Posts: 59Trusted User
    edited November 2010
    Yes, both sites provide projection information.

    **************************
    USGS DRGEs (from GeoCommunity):

    Enhanced (collarless) digital raster graphics (DRG)

    Projection: UTM
    DATUM: NAD83 (WGS84 for the US Territories)

    A standard digital raster graphic (DRG) is a scanned image of a U.S. Geological Survey (USGS) standard series topographic map, including all map collar information. The image inside the map neatline is georeferenced to the surface of the earth and fit to the Universal Transverse Mercator projection. The horizontal positional accuracy and datum of the DRG matches the accuracy and datum of the source map. The map is scanned at a minimum resolution of 250 dots per inch.

    These enhanced DRGs have the collars removed. The removal of the collar information allows the Enhanced DRGs to be edge-matched and displayed simultaneously in a Geographic Information System.

    *****************************
    USGS quad grids (also from GeoCommunity):

    projection Lambert Azimuthal Projection
    units meters
    major axis 6370997.00000
    minor axis 0.00000
    radius of the sphere of reference 6370997.00000
    longitude of center of projection -100 0 0.00000
    latitude of center of projection 45 0 0.00000
    false easting 0.00000
    false northing 0.00000

    *******************
    I load the DRGEs first - they load without being prompted for projection information.
    Then I load the USGS quad 24k grid. It prompts me for projection data, which I enter as follows (the asterisked items):

    projection Lambert Azimuthal Projection *****
    units meters
    major axis 6370997.00000
    minor axis 0.00000
    radius of the sphere of reference 6370997.00000
    longitude of center of projection -100 0 0.00000 *****
    latitude of center of projection 45 0 0.00000 *****
    false easting 0.00000
    false northing 0.00000

    I'm currently looking at the DRGEs around Spokane WA:

    o47117e2 o47117e3 o47117e4 o47117e5
    o47117f2 o47117f3 o47117f4 o47117f5
    o47117g2 o47117g3 o47117g4 o47117g5

    There eventually will be others.

    Thanks,
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2010
    What datum are you selecting with the Lambert Azimuthal Projection? It sounds like from the description that it should be the 'Clarke 1866 Authalic Sphere' datum rather than NAD83 like the individual DRGE files use.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulp575paulp575 Global Mapper User Posts: 59Trusted User
    edited November 2010
    What datum are you selecting with the Lambert Azimuthal Projection? It sounds like from the description that it should be the 'Clarke 1866 Authalic Sphere' datum rather than NAD83 like the individual DRGE files use.

    I was selecting the NAD83 datum.
    Upon receipt of your reply, I redid it and used the Clarke 1866 Authalic Sphere and that did work.

    Now my question is - how did you figure that out from the information I provided? Or was that hidden somewhere and I missed it?
    I'm guessing that it's just something you knew!

    I'd like to figure out how to do this without having to ask every time!

    Thanks for your excellent support!
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited November 2010
    I saw that the projection definition listed a sphere of radius 6370997, which I knew was the same as the Clarke 1866 Authalic Sphere datum. I had also encountered this before so it triggered me knowing what to look for.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulp575paulp575 Global Mapper User Posts: 59Trusted User
    edited January 2011
    Problem is back - but not necessarily!

    Here's when the problem returns:

    1. Load USFS ESRI Exchange file (e00 format). Not sure of projection so I set it at UTM Zone 11 (120 degrees - 114 degrees) since I know I'll be loading USGS topoquads later.
    2. Load a couple of other USFS shape files; projection not needed as GM12 found it.
    3. Load Idaho county layer; GM did not ask for projection.
    4. Load a couple of USGS topoquads; again GM did not ask for projection.

    Now the problem!

    5. Load USGS quad grids (24k). Didn't know projection, you provided the following information (above):

    Projection : Lambert Azimuthal Projection
    Datum : Clarke 1866 Authalic Sphere
    longitude of center of projection : -100 0 0.00000
    latitude of center of projection : 45 0 0.00000

    24k quads are not even close!

    I think the problem is because 'Lambert Azimuthal Projection' is not listed in GM v12.01. I do see 'Lambert Azimuthal EqualArea' but not sure if that's the same or not; used it because it was the only one close.

    However, if I load the 24k quads first, everything seems to line up.

    Problem with that is I don't always know if I need the 24k quad layer or not.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2011
    When you load the 24k quad layer, what are you selecting for the units? It looks like maybe you selected feet when it should be meters or something like that.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulp575paulp575 Global Mapper User Posts: 59Trusted User
    edited January 2011
    When you load the 24k quad layer, what are you selecting for the units? It looks like maybe you selected feet when it should be meters or something like that.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com

    Not sure, but I'm thinking it's feet because that's what the topoquads are? But not sure.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2011
    Per the web site http://data.geocomm.com/readme/usgs/grids.html, the units for the 24k grids file is meters, not feet, so make sure to select meters.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulp575paulp575 Global Mapper User Posts: 59Trusted User
    edited January 2011
    Per the web site http://data.geocomm.com/readme/usgs/grids.html, the units for the 24k grids file is meters, not feet, so make sure to select meters.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com

    That fixed it.

    So you can mix feet with meters?
    I thought (obviously wrong!) that the units have to be the same.

    This can get very confusing for those of us who are not the experts!
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited January 2011
    Yes, you can mix any kind of units, projections, and datums, Global Mapper will convert everything to your current display projection. What is important is that you select the correct projection/datum/units for each file when loading it (if asked) so that Global Mapper can correctly reproject it.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • paulp575paulp575 Global Mapper User Posts: 59Trusted User
    edited January 2011
    Thanks!

    I'm slowly beginning to figure all this out - I think!
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