HowTo: Identify the spatial reference, projection, or coordinate system of data

Article ID:	29129
Software:	ArcGIS - ArcEditor 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1 ArcGIS - ArcInfo 8.0.1, 8.0.2, 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1 ArcGIS - ArcView 8.1, 8.1.2, 8.2, 8.3, 9.0, 9.1, 9.2, 9.3, 9.3.1
Platforms:	N/A

Summary

Instructions provided describe how to identify the spatial reference, projection, or coordinate system of data. GIS and Computer Aided Design (CAD) data are frequently received with no spatial reference information. Because ArcMap's 'project on the fly' utility requires that projections be defined so that the data aligns in ArcMap, the coordinate system of the data must be identified and defined.

Data can be created in one of three types of coordinate systems:

• Geographic
• Projected
• Local

Procedure

Before performing the steps in this article, review ESRI Knowledge Base article: "Projection Basics: What the GIS professional needs to know", available through the link in the Related Information section below.

Examine the coordinate extent of the dataset:

A. Start ArcMap with a new, empty map.
B. Click on the Add Data button, and add the data with the unknown coordinate system to ArcMap.
C. Right-click on the name of the layer > Properties > Source tab.
D. Count the number of digits to the left of the decimal in the Extent box on Top, Bottom, Left, and Right. Ignore any digits to the right of the decimal.
E. Make a note of the number of digits in each of these positions, including the '-' sign if the coordinates are negative.
F. Save this information as a reference to be used in the steps outlined below.
Geographic Coordinate Systems (GCS) use units of decimal degrees for coordinates. These units are often referred to as 'lat/long'.

Decimal degrees (DD) are angles, and these units of measure are often used with GIS data but rarely with CAD data.

Because there are 360 degrees in a circle, coordinates in DD can never be over three digits to the left of the decimal.

X-coordinates are Longitude values. For data in North America, the longitude values must be negative numbers between 0 and -180. Y-coordinates are Latitude values. For data in North America, the latitude values must be positive numbers between 0 and +90.

To view the distribution of geographic DD coordinates across the globe, click on the following -show me-

This is the distribution of positive and negative values for Longitude (X) and Latitude (Y) coordinates worldwide.

.

Data with coordinates in decimal degrees are in a Geographic Coordinate System (GCS). These data can be created on a vast number of different datums. The most commonly used datums in North America are North American Datum 1927 (NAD 1927), North American Datum 1983 (NAD 1983), and World Geodetic Survey 1984 (WGS 1984). To define the coordinate system for data in a Geographic Coordinate System, the correct GCS must be selected.

A list of supported GCS and datums is linked to the article "What Geographic Coordinate System or datum should I use for my data?" in the Related Information section below.

For more information about datums, see ESRI Knowledge Base article "What do the terms geoid, ellipsoid, spheroid and datum mean, and how are they related?" in the Related Information section below.
Both GIS and CAD data can be created using Projected Coordinate Systems (PCS). A wide variety of predefined PCS, using different units and datums, are installed with ArcGIS. In the United States, the most commonly-used Projected Coordinate Systems are State Plane and universal transverse Mercator (UTM). Most often data projected to these coordinate systems has an extent with six to eight digits to the left of the decimal.

For instructions on determining the projection for data in a Projected Coordinate System, click on the link to ESRI Knowledge Base article "Identify an unknown projected coordinate system using ArcMap" in the Related Information below.
CAD data is frequently created in a local coordinate system.

Unlike data in a Geographic Coordinate System that has its origin (0,0 coordinates), where the Prime Meridian crosses the Equator off the west coast of Africa, a local coordinate system can have its origin (0,0) anywhere on the surface of the earth.

A local coordinate system is most likely when the extent of the dataset has three, four, or five digits to the left of the decimal.

CAD data in a local coordinate system can be aligned with other data, in a Projected Coordinate System in ArcMap, by creating a custom projection file. Refer to the article "Create custom projection file in ArcMap to align CAD data" in the Related Information section below.

Related Information

Projection Basics: What the GIS professional needs to know
The following concepts are fundamental to understanding the use of map projections in ArcGIS. 1. Coordinate systems, also known as map projections, are arbitrary designations for spatial data. Their purpose is to provide a common basis for comm...
What geographic coordinate system or datum should be used for my data?
To determine which Geographic Coordinate System datum to use for data, select the appropriate Geographic Coordinate Systems by Area of Use link in the Related Information section below. This document lists Geographic Coordinate Systems (GCS) s...
What do the terms geoid, ellipsoid, spheroid and datum mean, and how are they related?
The {BOLD}geoid{/BOLD} is defined as the surface of the earth's gravity field, which is approximately the same as mean sea level. It is perpendicular to the direction of gravity pull. Since the mass of the Earth is not uniform at all points, and t...
Identify an unknown projected coordinate system using ArcMap
Instructions provided identify the projected coordinate system for datasets that do not have coordinate system information in the metadata or a projection definition, using the project-on-the-fly capability in ArcMap. Coordinate system (project...
Create custom projection file in ArcMap to align CAD data
Instructions provided are for creating a custom projection file for CAD data, to align the CAD file with other data in a projected coordinate system, which uses units of feet, meters or other linear units.{NOTE}These instructions should not be use...

Created: 4/18/2005
Last Modified: 1/21/2010