Projected Coordinate System vs. Geographic Coordinate System

[SeanGerhardt]

Hey everyone,

I'm new to the world of GIS so I was somewhat confused when I saw the option of choosing from either a geographic coordinate system or projected coordinate system in arcMap.

My first layer is a Landsat 5 image that reads as being in WGS_84_UTM_zone_15_N, which is a projected coordinate system in arcMap. I am trying to display a shapefile over this Landsat image, but the shapefile is in WGS84 which is a geographic coordinate system in arcMap. The input for the shapefile is in lat/long without regard to projection.

When I load the shapefile layer, arcMap gives me a warning that the coordinate systems differ, but the shape file still appears to be in the correct place.


So my main questions are:

1. What is the difference a geographic coordinate system and a projected coordinate system?

2. The shapefile is in WGS_84 and the Landsat 5 image is in WGS_84_UTM_15. Will this incorrectly affect the placement of the shapefile, or can I simply ignore this warning?

Any help would be greatly appreciated!

[MargaretMaher]

Responding to Dennis Patterson's post today, ArcMap does not apply any Geographic (datum) transformations automatically.  When the warning appears about the Geographic Coordinate System differing from other data in the map, a Geographic (datum) transformation must be selected by the user.  The best resource for selecting the "best" transformation method is the list linked to Knowledge Base article 21327.  This provides the names of available transformations in alphabetical order, the WKID, the accuracy of the transformation in meters, a verbal description of the area of use, coordinates for the extent for which the transformation should be used, and the transformation method and parameters. 
21327 - Select the correct geographic (datum) transformation when projecting between datums

From the description in the case above, it sounds like the imagery did not have its' projection correctly defined.  Could have been in decimal degrees in a Geographic Coordinate System, but defined as a Projected Coordinate System by mistake.

[DennisPatterson]

Margaret,

Thanks for the information.  I am still new to GIS so I am still figuring things out.  I was assuming the transformation happened automatically.  After looking into this I figured out why I thought this.  The datum's were so close it was hard to really see they were off.  I was thinking it was happening automatically since it looked correct.  I have since had instances where I did not select a transformation and it was way off.  I now know this needs to be done every time.  I learn something every day.

[CarterEdgeworth]

Ponder a geographic system as some smelly sphere and angles. UTM is planar so a Cartesian grid. The latter could be any number of unit types.

If you want to be very precise use UTM if  you want to be in the vicinity use geographic.