Differences in map projections

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08-04-2010 11:23 AM
SteveFudge
New Contributor
I have a map layer of facility locations (points) in the Lambert Conformal Conic projection.  I used the buffer tool to place a 3 mile buffer around each facility.  I also have a layer of census block centroids that is in the same projection.  I intersected the buffers with the census blocks to determine which blocks are within 3 mi of a facility.  This worked fine.  Next, I took the census blocks that are within 3 mi of a facility and projected from Lambert to UTM zone 10 (facilities are in Washington state).  Within the UTM coordinate system, I see that some of the census blocks that were barely within the 3 mile buffer in Lambert are now just outside of it.  For example, in Lambert one census block was 2.999 miles from the facility, but in UTM 10 it is 3.18 miles away.  I know there differences in the projections and perhaps the datums, but this seems like a big error.  Did I do something wrong?  What kind of differences should I see between Lambert Conformal and UTM in the western part of the U.S.?

Thanks
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6 Replies
JohnSobetzer
Frequent Contributor
You might check out the following: http://blogs.esri.com/Support/blogs/mappingcenter/archive/2009/07/15/The-Buffer-Wizard-in-ArcMap.asp....

You might Google for articles like this as to differences in projections, although this one is pretty general: http://egsc.usgs.gov/isb/pubs/MapProjections/projections.html.

Related to this is http://blogs.esri.com/Dev/blogs/defenseandintelligence/archive/2010/05/24/Geodesic-features-and-meas... in case you wanted to check your results with the measure tool.
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MelitaKennedy
Esri Notable Contributor
I have a map layer of facility locations (points) in the Lambert Conformal Conic projection.  I used the buffer tool to place a 3 mile buffer around each facility.  I also have a layer of census block centroids that is in the same projection.  I intersected the buffers with the census blocks to determine which blocks are within 3 mi of a facility.  This worked fine.  Next, I took the census blocks that are within 3 mi of a facility and projected from Lambert to UTM zone 10 (facilities are in Washington state).  Within the UTM coordinate system, I see that some of the census blocks that were barely within the 3 mile buffer in Lambert are now just outside of it.  For example, in Lambert one census block was 2.999 miles from the facility, but in UTM 10 it is 3.18 miles away.  I know there differences in the projections and perhaps the datums, but this seems like a big error.  Did I do something wrong?  What kind of differences should I see between Lambert Conformal and UTM in the western part of the U.S.?

Thanks


Hi Steve,

I'm not really surprised, and there are several things that could affect this.

Neither projection maintains distances. They're both conformal which maintain shapes. A UTM zone is designed to have distortions no worse than 1 part in 2500: thus the 0.9996 scale factor parameter.

It also depends on the parameter settings in the Lambert conformal conic projection. The data may be in a different location relative to the projection's standard lines (where scale is 1.0) than when using UTM.

Another problem may be due to the Buffer tool creating a circle with circular arcs rather than a densified polygon. When projecting a circular arc, we maintain it as a circle. It's possible that the buffer would change more if it was composed of densified points.

Melita
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SteveFudge
New Contributor
Thanks for the replies.  The comment about the facility location in relation to the Lambert standard parallel lines versus UTM makes sense.  I believe the North American Lambert Conformal Conic projection I'm using has standard lines that are centered somewhere in the middle of the U.S.  My facility locations are in Washington state where Lambert is more distorted.

Can ArcGis create a buffer of densified points?  How is that done?

Thanks.
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WilliamClark
New Contributor II
I would reproject everything into an equal area projection (Albers perhaps) and modify the standard parallels of the projection if necessary.
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JonathanBoright
New Contributor II
I would reproject everything into an equal area projection (Albers perhaps) and modify the standard parallels of the projection if necessary.


Hi Wilderness Is there a reason why you would use an equal area instead of an equal distance projection?  I'm running into a similar problem.

Also, when I export buffers created in ArcMap to Google Earth (.kmz) I find that the distances as measured by Google Earth's distance tool to be significantly different. Any Idea how accurate Google Earth is at distance measures?

Cheers

Jon
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WilliamClark
New Contributor II
Jon,
Equal distance is good too depending on your application; I guess in my applications I've been more concerned of the area taken up by the buffer.  Map scale is a factor also.
I don't know of Google's precision.
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