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Today’s Objectives
name two types of coordinate systems identify components of each type of coordinate
system assign coordinate system information to a dataset set display units for a data frame and measure
distances on a map explain what a map projection is list the major categories of map projections list spatial properties that may be distorted when
different map projections are applied change the map projection for a data frame and
describe its effects
Coordinates in ArcGIS All geographic data have geographic
coordinates (lat/lon) Some may have projected
coordinates in addition to the geographic ones
ArcGIS assigns the coordinate system to a map based on the GCS or PCS of the first dataset loaded
Subsequent datasets are converted on-the-fly
Map and Display Units
Map units are determined by GCS or PCS GCS in degrees or decimal degrees PCS usually in feet or meters
Display units are determined by you They are defined as part of the data
frame
(Decimal) Degrees
Converting from degrees to decimal degrees1. Divide each value by the number of
minutes (60) or seconds (3600) in a degree
2. Add up the degrees to get the answer
Understanding Distortion
Distortion cannot be avoided; we have to choose from distortion of Shape
Area
Distance
Direction
Preserving Properties If two properties are to be preserved
then one is always direction
These properties are incompatible:
Classifying Projections
Cylindrical straight parallels; straight meridians
Pseudo-cylindrical
straight parallels, curved meridians
Conicpartial concentric circles for parallels; straight meridians
Pseudo-conicpartial concentric circles for parallels; curved meridians
PlanarConcentric circles for parallels; straight meridians
Modified planarNo common appearance of parallels and meridians
Choosing a Map Projection
Conformal (shape-preserving) maps Topographic and cadastral Navigation Civil engineering Weather
Choosing a Map Projection
Scale-preserving mapsno map preserves true distance for all measurements Airline distances Distance from
epicenter of an earthquake
Cost calculations
Choosing a Map Projection
Characteristics of Map Projections
Projection Category
Properties Common Uses
Conformal Preserves local shapes and angles
Topographic maps, navigation charts, weather maps
Equal Area Preserves areas Dot density maps, thematic maps
Equidistant Preserves distance from one or two specified points to all other points on the map
Maps of airline distances, seismic maps showing distances from an earthquake epicenter
Azimuthal All directions are true from a single specified point (usually the center) to all other points on the map
Navigation and route planning maps
Compromise No point is completely distortion free; distortion is minimized near the center and along the equator
World maps
UTM Projections Each zone uses a custom Transverse
Mercator projection with its own central meridian
SPC
N-S zones use Transverse Mercator E-W zones use Lambert Conformal
Conic Maximal scale error is 1:10,000 NAD27 or NAD83 datum
PLS It is used to locate areas, not points
It is not rigorous enough for spatial analysis like the calculation of distance or direction
It is not a grid imposed on a map projection (a system invented in a room), but lines measured on the ground by surveyors
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