Working with Coordinate Systems in ArcGIS

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Working with Coordinate Systems in ArcGIS

• Amanda Henley
• GIS Librarian
• Davis Library Reference

2
Modeling the Earth

• The Earth is not a perfect sphere
• It is an Oblate Spheroid
• Different Spheroids have been devised to model
  the earth- they are distinguished by the length
  of their axes

3
Geographic Coordinate Systems

• Locations are defined on a 3-D spherical surface
• Made up of graticules rather than grid cells
• Units are in degrees

Image Source Understanding Coordinate Systems,
ESRI 2000 http//its.unc.edu/gis/arcgis/pc_docume
ntation_83/Understanding_Map_Projections.pdf
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Geographic Coordinate Systems

• Not uniform
• Distances and measures are not accurate
• Meridians Converge Near Poles
• 1 longitude
• _at_ Equator 111 km
• _at_ 60 lat. 55.8 km
• _at_ 90 lat. 0km

Distance of 60 long at equator vs. Distance of
60 long at 40 latitude
Original Image Source www.learner.org/jnorth/tm/D
istanceLatLong.html
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Geographic Coordinate Systems

• Use Decimal Degrees (angles), 3 digits or less
• North America
• West of the Prime Meridian, so Longitude (X) is
  negative
• North of the Equator, so Latitude (Y) is positive
• Geographic Coordinate Systems include
• A Datum
• An angular Unit of Measure (degrees)
• A Prime Meridian

Image Source http//support.esri.com/index.cfm?f
aknowledgebase.techarticles.articleShowd29129.
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Datums

• Link a spheroid to a location on the earth
• Define the origin and orientation of the
  coordinate systems used to map the earth

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Geodetic Datums

• There are many datums
• Local
• NAD 27 Datum, uses Clarke 1866 spheroid
• NAD 83 Datum, uses GRS 1980 spheroid
• Global
• WGS 84 Datum, uses WGS 1984 spheroid
• In addition to being in the same projection,
  data themes must also be in the same datum.

Source Peter H. Dana, The Geographer's Craft
Project, Department of Geography, The University
of Colorado at Boulder http//www.colorado.edu/g
eography/gcraft/notes/datum/datum_f.html Accessed
01/25/06
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Datum Errors May Be Difficult to See
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Datum Transformations ArcGIS 9.2

• NAD 1927 to NAD 1983
• -for areas in the 48 contiguous states
• Name Code Area of Use
• NAD_1927_To_NAD_1983_NADCON 1241 United States
  (contiguous 48 states - CONUS)
• WGS 1984 to NAD 1983
• -for areas in the 48 contiguous states
• Name Code Area of Use
• NAD_1983_To_WGS_1984_5 1515 United States
• There are many transformations to chose from,
  each is appropriate for different areas.

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Data in a Geographic Coordinate System
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Projected Coordinate Systems

• Projected Coordinate Systems mathematically
  transform the 3 dimensional earth so that it can
  be modeled in 2 dimensions.
• This results in distortion
• Different projections are used for different
  areas and purposes

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Map projections Distortion

• Converting from 3-D globe to flat surface causes
  distortion
• Types of distortion
• Shape
• Area
• Distance
• Direction
• No projection can preserve all four of these
  spatial properties
• If some properties are maintained,
• errors in others may be exaggerated

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Families of Projections

• Planar/Azimuthal
• Cylindrical
• Conical

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Commonly Used Projected Coordinate Systems

• State Plane- a coordinate system that divides
  the United States, Puerto Rico and U.S. Virgin
  Islands into gt120 zones.
• North Carolina State Plane Meters, NAD83 is used
  by the North Carolina CGIA
• NC State Plane Feet, NAD83 is used by most local
  data providers.

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U.S. State Plane Zones
Image Source http//www.warnercnr.colostate.edu/c
lass_info/nr502/lg3/datums_coordinates/spcs.html,
Accessed January 27, 2006.
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Georgia State Plane Meters East and West
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North Carolina State PlaneMeters and Feet
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Commonly Used Projected Coordinate Systems Cont.

• UTM- Universal Transverse Mercator divides the
  globe into 60 zones, each 6 longitude.
• Transverse Mercator is accurate for narrow zones
• Often used for large scale scientific mapping
• Units are in meters

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UTM Zones in North Carolina
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Spatial Data for Orange County, NC in two
different UTM Zones
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Commonly Used Projected Coordinate Systems Cont.

• Albers Equal Area Conic
• Used by USGS for maps showing the conterminous
  United States (48 states) or large areas of the
  United States. Well suited for large countries or
  other areas that are mainly east-west in extent
  and that require equal-area representation. Used
  for many thematic maps.

Source USGS http//erg.usgs.gov/isb/pubs/MapProje
ctions/projections.html Accessed, 1/27/06.
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Albers Equal Area Conic
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Working With Coordinate Systems in ArcGIS

• On the Fly Projection
• Datum Transformation
• Defining a Layers Coordinate System vs.
  Projecting Data