Title: Representing the earth on maps: projections and coordinate systems
1Representing the earth on maps projections and
coordinate systems
- J426 Class 5
- January 27, 2009
2Overview
- Longitude and latitude
- Map projections
- Projected coordinate systems
3Placing the geographic features on the map
- Geographic locations measured using latitude and
longitude - Latitude
- Degrees north or south of equator
- North and south poles 90 degrees
- Longitude
- Degrees east or west or prime meridian running
through Greenwich, England - Maximum 180 degrees at (approximately)
International Date Line in Pacific
4Lines of latitude (horizontal) and longitude
(vertical)
5GIS and decimal degrees
- Degrees of latitude and longitude traditionally
measured in degrees, minutes, and seconds,
specifying north or south latitude, east or west
longitude - GIS and computers use decimal degrees, specifying
positive (north and east) or negative (south and
west)
6Positive and negative decimal degrees
7Location of Business/SPEA Building
8Further complications
- Specifying latitude and longitude dependent upon
shape of earth, which is an oblate spheroid - Model used for shape of earth affects
measurements - Specification of sea level affects measurements
- Complete specification called datum
- North American Datum 1927 (NAD 27)
- North American Datum 1983 (NAD 83)
9Map projections
- Latitude and longitude are coordinates on surface
of (approximately) a sphere - To represent 3-dimensional surface of earth on
2-dimensional map requires some form of
projection - Projections always involve some form of distortion
10World Mercator projection
11World Mollweide projection
12Properties of projections
- Projections can preserve some aspects of
relationships on map while distorting others - Conformal projections preserve directions and
shapes for small areas - Equal area projections preserve areas
- Equidistant projections preserve distances from
one or two points
13United States Lambert conformal conic projection
14United States Albers equal area projection
15Choice of projection
- For maps used for navigation, want projection
that preserves direction, e.g., Mercator - For other purposes, want projection that
preserves distance - For maps focusing on areas of different subareas,
like thematic maps, want projection that
preserves area
16Choice of projection (continued)
- Projection used makes greater difference on
smaller-scale maps - Maps of world vary drastically with projection
- Maps of United States vary significantly with
projection - Errors brought about by projection are greatest
at smallest scales
17Choice of projection (continued)
- Projection used makes less difference on
larger-scale maps - Maps of Central Indiana vary little with
projection used - Projection introduces very little error in
large-scale maps
18Projections in ArcGIS
- In ArcGIS, data in decimal degrees can be
displayed unprojected, using latitude and
longitude as x- and y-coordinates - Leads to vertically-squashed appearance of maps
- Data in decimal degrees can also be projected to
other projections for display
19Map of Indiana counties in decimal degrees,
unprojected
20Projections in ArcGIS (continued)
- Files in ArcGIS can also be in projected
coordinates - These files can likewise be projected to other
projections for display - Coordinates of first file added to map determines
default projection (but can be changed)
21Projected coordinate systems
- For smaller areas, in which errors associated
with projection are small, sometimes more
convenient to measure locations on earth in
rectangular coordinate system rather than in
degrees latitude and longitude - Involves establishing projection, determining
rectangular coordinate system in this projection,
measuring in feet or meters from origin using
these coordinates
22Projected coordinate xystems (continued)
- In U.S., State Plane coordinate systems used for
much local data collection - Each state has one or more State Plane coordinate
systems - Indiana East (most often used for Marion County)
- Indiana West
- State Plane systems can use either NAD27 or NAD83
23State Plane coordinate system areas
24Universal Transverse Mercator (UTM) coordinate
system
- Universal Transverse Mercator system divides
world into slices having width of 6 degrees
longitude - Rectangular coordinate systems for each numbered
zone - Indiana is in Zone 16
- Used on USGS topographic maps
- Similar system used by U.S. military
- Smaller countries have national coordinate systems
25Universal Transverse Mercator (UTM) zones in
United States
26U.S. National Grid coordinate system
- Based upon UTM coordinate system
- Further subdivides zones into square grid areas
- Standard system for identifying grids,
coordinates within grids - Being advocated by U.S. Department of Homeland
Security for all emergency operations
27Importance of understanding projections even when
doing large-scale maps for small areas
- Suppose you are displaying map with UCR crime
data, other layers, using State Plane Indiana
East NAD 1983 coordinates - Download census tract boundaries from web to
include on map - Convert from decimal degrees to State Plane
Indiana East but choose NAD 1927
28Result after adding census tract boundaries to map
29Result after zooming out to display all