intro to gis | summer 2011 map projections
TRANSCRIPT
PCS
• Translates geographic coordinate system to a flat plane
• Transforms lat/long geographic coordinates to x/y
(Cartesian) coordinates using mathematical formulas
• Varying distortions of shape, area, distance, and
direction always result from this process
Projection Properties
• Conformal: preserves shape at the expense of area &
distance; meridians & parallels intersect at right
angles;
• Equal Area: preserves area at the expense of shape;
mostly used for analysis
• Equidistant: preserves distance at the expense of
shape & size; equal area projections can also be
equidistant
Universal Transverse Mercator (UTM)
• UTM zone numbers designate 6° longitudinal strips
from 80° south latitude to 84° North latitude
• UTM coordinates are in “northings” and “eastings”
(meters)
• “northings” are relative to the equator; “eastings” are
relative to false origin 500,000 meters west of the
zone’s central meridian
State Plane
• Developed in the 1930’s to provide local reference systems
that were tied to a national datum (NAD 83 or NAD 83
HARN)
• Projections are chosen to minimize distortion based on the
state’s shape (good for surveying)
• Feet or meters
• Smaller states use a single state plane zone; larger states are
divided into several zones
• NOT a projection; system is based on different projections
Lambert Conformal Conic
Albers Equal-Area Conic
Standard Parallels 30 N & 46 N
Standard Parallels 33 N & 45 N
Geographic Coordinate Systems
• Latitude/longitude system based on a particular datum
(i.e., NAD 83)
• In ArcGIS, named “GCS_{datum name}”
• Lat/long is not a projection and therefore cannot be
shown on a flat surface (map, screen)
• Shown in ArcGIS as the “Platte Carree” projection
• Units = degrees, minutes, seconds or decimal degrees
Projected Coordinate Systems
• Method used to translate the GCS on to a flat plane
• In ArcGIS, named “{projection name}_{projection
type}_{datum name}”
• Uses real-world units (meters, feet)
Define Projection tool
• When the data does not have a defined projection or
coordinate system (no .prj file)
• Term “projection” is misleading (why?)
• Projection or coordinate system can be created,
selected from a predefined list, or imported from
another dataset
Project tool
• When you want to change the defined projection or
coordinate system
• If the geographic coordinate system changes, requires a
geographic transformation
• Projection or coordinate system can be created,
selected or imported
• Creates a new dataset (user specifies name & storage
location on computer)
ArcMap Data Frame
• Takes on the projection or GCS of the first dataset
added
• All other datasets are then projected “on-the-fly” to
match it (assuming they have a defined projection)
• The data frame’s projection/GCS can be set manually in
the data frame “Properties”
• When you change the projection or GCS of the data
frame, it changes to appearance of the map data
ArcMap & Projections
• GIS data files & data frames (in an .mxd) need to have
defined projections
– GIS data files have a .prj file that stores projection info
• GIS data MUST be defined in the correct projection,
which is the projection used to create the data (GPS,
Satellite image, digitizing from a paper map, etc.)
– If downloading data, it will either have a .prj file that
saves the projection information OR the projection
info is available in the documentation on website