Take a look at the attached image from ArcMAP. These are two sets of GPS collected point data that should be more or less coincident.
The green points on the right are in the correct location, the purple points to the left are not. You can see the simialr shape in extent and relative locations. You can also see from the attached table pics that both are in geographic coordinates.
These points were collected in the field with a GPS dongle on a laptop or net book. The field staff are not sure about what coordinate systems were used during that time (!) but for the green data WGS 84 seems to be correct.
*My question is, can you tell by looking at the small difference in location and scale of the purple points what kind of error is occurring here? *
From me experience, when the wrong projection has been defined the points will be WAY off, not just a little. Especially if defined as geographic when it should be projected etc. But in this case I know it's geographic based on what I see for coordinates in the table.
On the other hand it seems too far off to be a datum shift.
I've tried defining a a few general geographic coordinate systems but with no change in location.
The researcher I'm helping with this project will be contacting the field staff to see if they can verify the methodology and setting for collection. In the mean time I'm hoping you guys can help me figure out what's going on here.
Thanks,
David
The green points on the right are in the correct location, the purple points to the left are not. You can see the simialr shape in extent and relative locations. You can also see from the attached table pics that both are in geographic coordinates.
These points were collected in the field with a GPS dongle on a laptop or net book. The field staff are not sure about what coordinate systems were used during that time (!) but for the green data WGS 84 seems to be correct.
*My question is, can you tell by looking at the small difference in location and scale of the purple points what kind of error is occurring here? *
From me experience, when the wrong projection has been defined the points will be WAY off, not just a little. Especially if defined as geographic when it should be projected etc. But in this case I know it's geographic based on what I see for coordinates in the table.
On the other hand it seems too far off to be a datum shift.
I've tried defining a a few general geographic coordinate systems but with no change in location.
The researcher I'm helping with this project will be contacting the field staff to see if they can verify the methodology and setting for collection. In the mean time I'm hoping you guys can help me figure out what's going on here.
Thanks,
David