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LOL hang on to your cowboy hat Sean. You just asked one of the most important questions any new GIS user wants to know about, and the answer is simple and complicated all at once. There are many people here much more knowledgeable than me, so I will let them answer question #1. Question #2, Arc will project different spatial references "on the fly" (provided they are all correctly defined) so you don't need to worry on that score. It is best to have everything in the same coordinate reference system so Arc doesn't use up a lot of processor doing that, but not required.
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!
I might suggest looking over the first topics in the geographic coordinate systems and projected coordinate systems 'chapters' in the help. Here's a link to the GCS one:
What are geographic coordinate systems
In the installed (or online) help, the map projections section (in v10) is in Professional Library, Guide books.
When displaying data that's using a geographic coordinate system, ArcMap uses a 'pseudo-Plate Carree' projection. Basically, we just treat the coordinate values as if they're linear and just display the data.
When you added the two datasets to ArcMap, I believe you got the geographic coordinate systems (datums) are different warning message. Even though we can see that both datasets are using WGS84 as the model of the earth, the software does strict checking when comparing coordinate systems. I think the names of the two WGS84 definitions are different enough that the warning message is called. You can ignore it.
Hi Sean - I thought I would add my two cents. Like @mytmatt metioned, coordinate systems/projections is not only an important question but one of the most important aspects of GIS. Hopefully this will help you out some:
What Is A Geographic Coordinate System?
You can think of a Geographic Coordinate Systems as data that is defined by a 3-D surface and measured in latitude and longitude. An example of a Geographic Coordinate System would be "WGS 1983" or "North American Datum 1983". You may also wonder what a "Datum" is. Just remember that the term "Datum" and "Geograhpic Coordinate System" can be used interchangeably. Essentially a Datum provides a "frame of reference for measureing locations on the surface of the earth i.e. lines of latitude and longitude."
What is a Projected Coordinate System?
A projected coordinate systems refers to data that is defined by a flat 2-D surface and can be measured in units of meters and feet. An example would be USA Albers Equal Area Conic which has a measuring unit of Meters. "Map projections" and "Projected Coordinate Systems" can be used interchangably as well.
That is a very simplistic description of the differences between "Geographic Coordinate System" and "Projected Coordinate System" but one that should at least give you some of the basics of what you wanted to know. Adding to what Melita wrote I thought I would add a couple extra Resource Center links that might help you..
What Are Map Projections?
Projection Basics for GIS Professionals