Projected Coordinate System vs. Geographic Coordinate System

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02-28-2011 01:52 PM
SeanGerhardt
New Contributor II
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!
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12 Replies
MattMorris
New Contributor III
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.
MelitaKennedy
Esri Notable Contributor
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!


Hi,

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.

Melita
MatthewNordhagen
Occasional Contributor
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?
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/What_are_map_projections/003r000000010...

Projection Basics for GIS Professionals
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Projection_basics_for_GIS_professional...
DennisPatterson
New Contributor II

Mr. Nordhagen,

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.

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/What_are_geographic_coordinate_systems...

Dennis Patterson

General Dynamics

Digital Training Instructor

AdrianWelsh
MVP Notable Contributor

Sounds like Margaret Maher​ might need to chime in on this talk! 

She is the ultimate expert in coordinate systems.

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AbdullahAnter
Regular Contributor II

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).

AND

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)

NaveedAli
New Contributor

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.

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MargaretMaher
Esri Contributor

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!@

DennisPatterson
New Contributor II

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. 

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