1. Map Projections & Coordinate Systems

2. What is a map projection?
Because the earth is spherical and maps are flat, GIS applications require that a mathematical formulation be applied to the earth to represent it on a flat surface.
This causes some distortions of distance, area, shape, or direction.

3. Map Projections - Distortion
preserve shape - conformal (orthomorphic)
preserve scale – equidistant
preserve direction – azimuthal
preserve great circles - gnomic
preserve circular shapes - stereographic

4. What are map projections?
There are two types of coordinate systems
Geographic – use latitude and longitude coordinates on the surface of a sphere
Projected – use mathematical conversion to transform latitude and longitude coordinates to a flat surface.

5. ArcMap has over a hundred projections

6. Albers – equal area conic projection
Shape and linear scale
Minimally distorted in the region between the standard parallels
Areas
proportional to the same areas on the earth
Direction
Locally true
Distance
Most accurate in the middle latitudes

7. Robinson – world projection
Compromise projection used for world maps
Shape
Distortion is very low within 45° of the origin and along the equator
Areas
Distortion is very low within 45° of the origin and along the equator.
Direction
Generally distorted
Distance
Scale is constant along any given latitude and for latitude of the opposite sign
Limitations
Useful only for world maps

8. State Plane
Not a projection it is a coordinate system that divides the fifty U.S. states, Puerto Rico, and the U.S. Virgin Islands into more that 120 numbered sections, referred to as zones.
Designed for large-scale mapping in the U.S.

9. UTM – Universal Transverse Mercator
Shape
Accurate representation of small shapes
Areas
Minimal distortion within each UTM zone
Direction
Local angles are true
Distance
Scale is constant along the central meridian
Limitations
Error and distortion increase for regions that span more than one UTM zone

10. Map Projections vs. Datum Transformations
A map projections is a systematic rendering from 3-D to 2-D
Datum transformations are from one datum to another, 3-D to 3-D
Changing from one projection to another may require both

11. Datum To more accurately represent locations on the earth’s surface
A datum links a spheroid to a particular portion of the earth’s surface.

12. Datum Most commonly used datums in North America
North American Datum (NAD) 1927 using the Clarke 1866 spheroid
NAD 1983 using the Geodetic Reference system (GRS) 1980 spheroid
World Geodetic System (WGS) 1984 using the WGS 1984 spheroid
Input geographic
coordinate system
NAD 1927
Output geographic
coordinate system
WGS 1984

13. Datum
The coordinates for a location will change depending on the datum and spheroid on which those coordinates are based.

14. Georeferencing – information is tied to a specific location on the earth's surface

15. Georeferencing – what?
Aligning geographic data to a known coordinate system so it can be viewed, queried, and analyzed with other geographic data
Georeferencing may involve shifting, rotating, scaling, skewing, and in some cases warping, rubbersheeting, or orthorectifying the data

16. Georeferencing – why?
Raster data is commonly obtained by scanning maps or collecting aerial photographs and satellite images
Scanned map datasets don't normally contain spatial reference information
aerial photography and satellite imagery, sometimes the location information delivered with them is inadequate and the data does not align properly with other data you have. Thus, to use some raster datasets in conjunction with your other spatial data, you may need to align, or georeference, to a map coordinate system.

17. Georeferencing - Transformation
Coordinates in the source system
Coordinates in the target system