Difference between projected and unprojected data

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11-03-2021 02:44 PM
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StephenKing3
New Contributor III

Hello,

I'm looking for a bit of advice regarding geographic coordinate systems and projected coordinate systems, and digitising data in ArcGIS Pro.

I have a Shapefile depicting certain zones (polygons) and the data uses WGS 1984 as the coordinate system.

I have digitised new features over part of these zones, snapping to some of the vertices. I have then added the original dataset to a new map which is projected to WGS 1984 UTM Zone 30 North and ArcGIS Pro reprojects the data. When I overlay the new dataset that I digitised the boundary no longer aligns to the original data in some areas of the feature, even though during digitising the new lines snapped to the underlying data.

Can anyone advise what is happening - is it a projection distortion?

Thanks.

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DanPatterson
MVP Esteemed Contributor

Consider the example of digitizing along a line of latitude in an east-west direction using geographic coordinates (aka, a GCS with a WGS-84 datum).  If you just pick a start and end point, then project the data, there is a chance that the intervening space will not follow that line of latitude.  To ensure that it approximates the line of latitude, when projected, you need to densify your point creation,

In short the distance between lines of longitude along lines of latitude decrease as you head poleward.  This is a fact about trying to approximate a sphere/ellipsoid/geoid in planar coordinates.

In your case, you might have been better off digitizing your map using the projected coordinate system rather than the unprojected (geographic) coordinate system


... sort of retired...

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DanPatterson
MVP Esteemed Contributor

Consider the example of digitizing along a line of latitude in an east-west direction using geographic coordinates (aka, a GCS with a WGS-84 datum).  If you just pick a start and end point, then project the data, there is a chance that the intervening space will not follow that line of latitude.  To ensure that it approximates the line of latitude, when projected, you need to densify your point creation,

In short the distance between lines of longitude along lines of latitude decrease as you head poleward.  This is a fact about trying to approximate a sphere/ellipsoid/geoid in planar coordinates.

In your case, you might have been better off digitizing your map using the projected coordinate system rather than the unprojected (geographic) coordinate system


... sort of retired...
StephenKing3
New Contributor III

Hi Dan,

Thank you, that helps clarify what I thought might be happening.

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