Layer Projection Issues

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02-10-2021 02:17 PM
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JohnWilkins
New Contributor II

I'm a new ArcGIS Pro user.  Just finished David Smith's textbook, Understanding GIS Pro.  Great book!

In my research, most of the shapefiles that I bring into the map from outside sources, mainly government sources (but also some institutional sources), are conveniently in GCS_WGS_1984, and usually do not have any unique projection needs associated with them that are suggested by their metadata.  My desire is to work in a WGS_1984 environment when possible and my project map is in WGS_1984 (see attachment).  A typical example is the shapefile in yellow.  It is perfectly aligned with the ArcGIS Pro World Imagery layer, based on my ground knowledge of the area.  However, I have one shapefile (symbolized in red) that is in WGS_1984, but with no projected coordinate system identified, so I’m assuming one is not needed for it in this case.  However, it does not align with the ArcGIS Pro World Imagery.  I’m obviously doing something wrong or remain a little confused about how to address projection issues.  Sri Lanka is in the WGS 84 UTM zone 44N.   There are some transformation Datums for Sri Lanka, (1) Kandawala_To_WGS_1984 and (2) SLD_To_WGS_1984_1; both suitable for Sri Lanka – onshore, and I have tried to do various transformations to try to improve the alignment, but nothing seems to work.  The fact that all three of the layers (including the map) I’m currently working with are in WGS_1984 would seem to indicate that there should be no alignment problem.  What am I doing wrong or do not understand about this projection problem, and what can I do to address it?

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DanPatterson
MVP Esteemed Contributor

WGS_84 per se isn't a coordinate system, it is a datum, which is why you can have

GCS_WGS_84  a geographic coordinate system with that datum, its coordinates are in decimal degrees

UTM zone 44N WGS 84  a projected coordinate with that datum, but its units are in meters and the projection is a universal transverse mercator projection, it is in zone 44 in the Northern hemisphere.

Don't get the little bits around the datum description.

If you find a file that doesn't have a "defined" coordinate system.  Don't use it

Do not under any circumstance use the Define Projection tool for anything else other than fixing the above, although you shouldn't use it unless you really know that it is in that coordinate system.

Use the Project tool to "make" a new file with a new coordinate system.

Also.... the first layer added to a map, defines its coordinate system, so add things properly.

Avoid any basemaps that use Web Mercator (the story is too long) especially if you intend to do any analysis.

Your questions are still being virus scanned, ..... but I couldn't wait ;)

 


... sort of retired...

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DanPatterson
MVP Esteemed Contributor

WGS_84 per se isn't a coordinate system, it is a datum, which is why you can have

GCS_WGS_84  a geographic coordinate system with that datum, its coordinates are in decimal degrees

UTM zone 44N WGS 84  a projected coordinate with that datum, but its units are in meters and the projection is a universal transverse mercator projection, it is in zone 44 in the Northern hemisphere.

Don't get the little bits around the datum description.

If you find a file that doesn't have a "defined" coordinate system.  Don't use it

Do not under any circumstance use the Define Projection tool for anything else other than fixing the above, although you shouldn't use it unless you really know that it is in that coordinate system.

Use the Project tool to "make" a new file with a new coordinate system.

Also.... the first layer added to a map, defines its coordinate system, so add things properly.

Avoid any basemaps that use Web Mercator (the story is too long) especially if you intend to do any analysis.

Your questions are still being virus scanned, ..... but I couldn't wait ;)

 


... sort of retired...
JohnWilkins
New Contributor II

Thank you.  I think I can figure it out.  Appreciate it.

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