Difference between projected and unprojected data

[StephenKing3]

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.

[DanPatterson · Accepted solution]

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

[StephenKing3]

Hi Dan,

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

[StephenKing3]

I am circling back to this topic because I have another related query.

I have a polygon for a lease area that was digitized in WGS84. I need to resize the polygon by moving a couple of the polygon's vertices, however, the remaining boundaries of the polygon must match the original. So I can do this in WGS84, but when the polygon is projected to UTM Zone 30 North, the corner vertices that were altered no longer match the original boundary. They are either outside or inside the boundary lines. How do I get the boundaries to stay the same after being projected to 30 North? The only way I can think of getting a close match is to use the 'Preserve Shape' option, but this adds extra vertices that I don't want.

[DanPatterson · Accepted solution]

preserve shape... would be what I recommend because they are needed to go from geographic to planar.

make a square in wgs84 decimal degrees.... project it withint preserve shape

now project it with preserve shape

conclusion? (lines of longitude and latitude are not straight lines to begin with)