Projection Process, Correct?
Roger Edrinn
Global Mapper UserTrusted User
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?
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
-
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 -
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.
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 -
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 -
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.
Thanks
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