Georeference Review - ArcGIS

(copied from Learning ArcGIS9 Virtual Campus course)

 

The accuracy of a GIS map or analysis depends on the accuracy with which feature locations are represented. A coordinate system is used to reference feature locations to the real world. Listed below are key points about coordinate systems that you should remember.

 

bullet

All geographic datasets have a geographic coordinate system (GCS).

bullet

There are different spheroids for different parts of the world, and there are different datums to help align the spheroid with the surface of the earth in different regions.

bullet

As long as they have a common geographic coordinate system, ArcMap can properly display multiple datasets in the same data frame using a process called "on-the-fly projection."

bullet

In ArcMap, the first layer you add to an empty data frame sets the coordinate system for the data frame.

bullet

You should use a map projection when you need to perform analysis, take accurate measurements, or when you need to show a particular spatial property as it really exists on the earth.

bullet

Anyone who uses maps should know which projections are being used and which properties are distorted and to what extent.

 

 

Review questions

  1. What are the components of a geographic coordinate system?

  2. Name one way you can get information about the coordinate system of a dataset in ArcCatalog.

  3. What are the two most important factors to consider when choosing a map projection?

 

Answers

  1. A geographic coordinate system is defined by a prime meridian (usually Greenwich), a datum (which includes a spheroid), and an angular unit of measure (degrees).

  2. You can use the Metadata tab to view the coordinate system information for a dataset. You can also look at the dataset's Properties dialog box.

  3. When choosing a map projection, you should consider the purpose of the map and which spatial properties you want to preserve.

 

Key terms

azimuth
coordinate system
datum
decimal degrees
display units
false easting
false northing
geographic coordinate system
graticule
map projection
map units
prime meridian
projected coordinate system
spheroid