Projection Process, Correct?

Roger Edrinn
Roger Edrinn Global Mapper UserTrusted User
edited April 2008 in Vector Data
I'm having problems with "identical" lines from different databases not being coincident. So I'm wondering if I'm handling projection correctly within Global Mapper?

My basic process:
Import a dataset, use the Options Dialog to set name and line type. Then use the Vector projection tab to set Geographic, NAD83. At this point I export a shapefile of the lines.

Later I'll create a workspace or catalog to assemble all of these files into a Master Project, ideally all in Geographic, NAD83.

A possible source of the error? One data set is in Geographic, NAD27 but when it opens in GM, GM says their is no projection info in the shapefile. Can I set NAD83 at this point? Or must I set NA27 and then convert to NAD83 using the projection tab?

Comments

  • global_mapper
    global_mapper Administrator
    edited April 2008
    Roger,

    To reproject data or convert datum, you do NOT want to change the projection/datum through the Options dialog for a layer. Rather you need to change it on the Projection tab of the Configuration dialog prior to export. Changing it on the Options dialog changes the projection/datum interpretation for a layer, which is used when an incorrect projection/datum is specified on load. The Projection tab of the Configuration dialog is used to actually convert projection/datums.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • Roger Edrinn
    Roger Edrinn Global Mapper User Trusted User
    edited April 2008
    global_mapper wrote: »
    To reproject data or convert datum, you do NOT want to change the projection/datum through the Options dialog for a layer.
    I'm not, I'm only setting the line style and name using the options dialog.

    However, what about this?
    One data set is in Geographic, NAD27 but when it opens in GM, GM says their is no projection info in the shapefile. Can I set NAD83 at this point? Or must I set NA27 and then convert to NAD83 using the projection tab?

    Thanks
  • global_mapper
    global_mapper Administrator
    edited April 2008
    Whenever you load a file you must select the projection/datum that the coordinates in the file are actually in. In your case you would select NAD27 for the datum at load, then change the datum on the Projection tab of the Configuration dialog later to reproject.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • Roger Edrinn
    Roger Edrinn Global Mapper User Trusted User
    edited April 2008
    global_mapper wrote: »
    Whenever you load a file you must select the projection/datum that the coordinates in the file are actually in. In your case you would select NAD27 for the datum at load, then change the datum on the Projection tab of the Configuration dialog later to reproject.
    This was the source of the problem. I went through the bulk of the files where this error might have crept in and redid them. Also, I was able to capture a discarded file that was in UTM, but zone 12? Didn't understand why the file was in another state. Set it to Zone 13 and low and behold, in Colo.

    Thanks

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