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.
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All geographic datasets have
a geographic coordinate system (GCS).
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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.
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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."
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In ArcMap, the first layer
you add to an empty data frame sets the coordinate system for the data
frame.
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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.
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Anyone who uses maps should
know which projections are being used and which properties are distorted
and to what extent. |
Review questions
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What are the components of a
geographic coordinate system?
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Name one way you can get
information about the coordinate system of a dataset in ArcCatalog.
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What are the two most
important factors to consider when choosing a map projection?
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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).
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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.
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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