GCS/projections & coordinates

07-18-2013 07:09 PM
New Contributor II
Hi All - I have an issue that has been nagging me forever. Here's an example. I have a layer in a map, and under Properties > Source it shows "Projected Coordinate System: NAD_1983_UTM_Zone_11N" and "Geographic Coordinate System: GCS_North_American_1983". My first question is: am I working with a projected, or a geographic coordinate system? Second: what is NAD 83? Third: is UTM a projected or geographic coordinate system? I'm so freaking confused! The more I look into it the less clear it becomes!

Lastly, when I open the attribute table, create a new field, field type double, etc... calculate geometry the only option I have is to calculate "X coordinate of point" or "Y coordinate of point". Why can't I calculate lat/long in minutes, seconds, etc? What if I wan't both in the same attribute table?

I'm thinking the answers to the first set of questions are huge, and that I know so little about it that I don't even really know how to ask the question. Any help would be greatly appreciated!
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Esri Notable Contributor

Yes, you have a lot of questions that have long answers! First, have you tried reading the map projections guide book in the help? There are several topics each on geographic and projected coordinate systems.

Geographic coordinate system: provides a frame work / reference system for locating positions on a spheroid or ellipsoid. Positions are located using angles and angular units. "Earth model". When displayed in 2D, it's actually projected using something like the Plate Carree map projection.

Projected coordinate system: Two-dimensional framework / reference system for location positions. Includes a GCS as part of its definition. Has to distort data to crush the 3D surface into 2D: distance, shape, area using a map projection and parameters customized for a particular area.

NAD 83 / NAD 1983 / North American Datum 1983: a GCS built using data from Canada, US, ?and Mexico? Finished in 1986 before GPS really got going, there have re-adjustments since in both Canada and the US. Canada: CSRS. US: HPGN / HARN, NSRS2007, and 2011.

UTM: Universal Transverse Mercator. A grid system designed by the military. It's divided into 6 degree wide zones (there are a few exceptions) and covers the entire world. The map projection is transverse Mercator, with similar parameter values for each zone. The poles have separate UPS zones that use the stereographic projection. Can be used with any GCS, although the military uses WGS84 almost exclusively.

DMS is a string format, not a double value. You can store only one value per double precision field, but can certainly add two fields to a table! You may need to set the data frame to a geographic coordinate system first. Also check out the Convert Coordinate Notation tool.

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MVP Regular Contributor
Projected and Geographic coordinate systems and coordinate system transformations can really drive you nuts... :confused: And yes, you are opening Pandora's box...

Nonetheless, I strongly recommend you to take serious time to familiarize yourself with the basic concepts, as the consequences of inappropriate and incorrect usage can be devastating (yes, not only on paper but also literally, I once worked a short period for an oil company, and they had a special introduction to coordinate systems for new employees working in GIS that started with an oil platform going up in flames due to wrong usage of coordinate system information - this was no joke!)

Read as much as you can from different sources, ultimately, things should start to clear up a bit.

As a starter:

The ArcGIS Help - What are map projections?

I also recommend you to read all other sub-topics under the same heading, look in the Help TOC (Table of Contents) on the left of the Online Help window for the sub-topics.

Also, this Ordnance Survey UK document, although partly focused on the UK situation, gives a nice introduction, and basic reminders and concepts, of Coordinate Systems:

A guide to coordinate systems in Great Britain

And these ITC webpages (some Dutch examples, ITC is an international institute but located in the Netherlands):


And the Wikipedia pages:


I can also recommend you these two pages by Eric Gakstatter, although you need to have some basic understanding first, and know some GPS jargon... See the Wikipedia links below.

Nightmare on GIS Street: Accuracy, Datums, and Geospatial Data

Part 2: Nightmare on GIS Street �?? Accuracy, Datums, and Geospatial Data

Wiki links:

Continuously Operating Reference Station

Differential GPS

Global Navigation Satellite Systems

Real Time Kinematic

Wide Area Augmentation System
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