Misaligned objects, not sure why
Roger Edrinn
Global Mapper UserTrusted User
Take a look at this screenshot and help me understand what's happening.
The orange and light grey roads were downloaded from the BTS roads database of the USGS as shape files. They were loaded into GM9 and assigned the NAD83 datum.
The black lines are from the US Forest Service roads database. When I loaded them into GM9, a popup appeared saying shapefiles have no datum, again I assigned NAD83.
This summer I noticed in the GPS that USFS trails and a recorded tracklog had a consistent offset where the trails were ~150 east of the tracklog, same as this screenshot shows. Also, their was very little offset when the trail went E-W, in only appeared on N-S trails.
I've concluded their are two possible reasons for the offset:
1] I don't know what I'm doing or
2] Their is a consistent error in the USFS dataset.
Someone please shed some light on what's happening. How do I fix the offset?
Thanks
The orange and light grey roads were downloaded from the BTS roads database of the USGS as shape files. They were loaded into GM9 and assigned the NAD83 datum.
The black lines are from the US Forest Service roads database. When I loaded them into GM9, a popup appeared saying shapefiles have no datum, again I assigned NAD83.
This summer I noticed in the GPS that USFS trails and a recorded tracklog had a consistent offset where the trails were ~150 east of the tracklog, same as this screenshot shows. Also, their was very little offset when the trail went E-W, in only appeared on N-S trails.
I've concluded their are two possible reasons for the offset:
1] I don't know what I'm doing or
2] Their is a consistent error in the USFS dataset.
Someone please shed some light on what's happening. How do I fix the offset?
Thanks
Comments
-
Are you certain that the data from the US Forest Service was in the NAD83 datum? Perhaps it is actually in the NAD27 datum. Try selecting that as the datum when you load the US Forest Service data and see if that helps.
Let me know if I can be of further assistance.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com -
global_mapper wrote: »Are you certain that the data from the US Forest Service was in the NAD83 datum?Perhaps it is actually in the NAD27 datum. Try selecting that as the datum when you load the US Forest Service data and see if that helps.
In any case, setting NAD27, substantially solved the misalignment . . . substantially but not perfectly. Which brings a follow on question:
How do I get rid of duplicate shapefile vectors? Easily?
The reason to use the USFS data is their roads go deeper, the last mile, so more info. The reason not to is the USFS does not identify their roads with a number or name unlike the BTS. So I only want the non-duplicate forest roads.
Thanks
Roger -
Roger,
The standard datum used for US data until recently (basically until the 90s) was NAD27, which is why many US data sets are still in that datum. Newer data typically uses NAD83 or WGS84 (which are basically identical).
There is not an easy way to automatically remove nearly identical vector features.
Let me know if I can be of further assistance.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com -
Found this by clicking on the Projection link at the USFS website:Projection GEOGRAPHIC
Units DD
DATUM NAD27
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