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!
You stated Geographic Coordinate System (GCS) and Datum terminology could be used interchangeably, but from the references I am reading it specifically states a datum is just one part of a GCS. The three parts are an angular unit of measure, a prime meridian, and a datum. I am not trying to sharp shoot, I am trying to ensure there has not been a change to the terminology, or if this is a commonly used expression in the community?
I am still trying to come up with my own dumbed down version of what a GSC vs Projected Coordinate System (PCS) is. I thought that your explanation was pretty good, but just wanted to see about the GCS/datum comment. I provided the link below that makes a reference to this in the very first paragraph.
Digital Training Instructor
I agree with you Matthew Nordhagen
Geographic coordinate system (GCS) Is used to define the shape of earth, that is mean the 3D of Earth ( Datum,Semimajor Axis and Semiminor Axis).
Projected coordinate system (PCS) is used to show the 3D earth into 2d flat sheet using projection methods.
so that (PCS) does not work without (GCS)
Thanks, Matthew It's a useful explanation. Now, I am much clear on both coordinates system. In a layman term, it's the measurement or plotting earth on a paper or a canvas in the computer either using 3D or 2D approach.
I appreciate Adrian's confidence in me, but Melita Kennedy (see her post above) has taught me everything I know about map projections. Melita is correct - ArcMap does project data in the fly, but if a single element in the Geographic Coordinate System (GCS) is just a hair different, as in a difference in the spelling, or the 13th digit to the right of the decimal, then ArcMap will ask for a Geographic Transformation even if one is not needed. To address another question above, a GCS consists of four elements - a datum; a spheroid, angular units of measure; and a Prime Meridian. I am also guilty of talking about "datum transformations" but these are actually "Geographic Transformations", since if any one of these elements is changed a transformation has to be applied - so it's not just a "datum transformation". Anyone following this thread may want to also check out my book "Lining Up Data in ArcGIS - a guide to map projections". FYI - I do NOT receive royalties, but think it will help our customers who are confused by projections like I was before Melita spent 2 1/2 years of her life sorting me out!@
I have found that transformations usually work, but if it does not work this is where your projection tool comes in handy. I had an issue that came up the other day that made me feel really stupid. I was dragging a shapefile on to a raster map layer of Afghanistan. I got the warning telling me a transformation was needed. I let the system conduct the transformation, but it did not appear to be working since the roads were way off when viewing the imagery. I attempted to use the projection tool, only to get the exact same result. I checked the datum, and it appeared to be the correct datum after the transformation and the projection tool were used. Both were providing the same results. In the end, I was so wrapped up in the datum transformation I forgot to take the scale of the imagery into account. The scale at which the imagery was taken was way too small for use with that road layer shapefile and when you zoomed to a larger scale the maps would appear to be way off. Needless to say I learned the hard way after spending a lot of wasted time figuring it out.