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.
4 What are map projections? There are two types of coordinate systemsGeographic – use latitude and longitude coordinates on the surface of a sphereProjected – use mathematical conversion to transform latitude and longitude coordinates to a flat surface.
6 Albers – equal area conic projection Shape and linear scaleMinimally distorted in the region between the standard parallelsAreasproportional to the same areas on the earthDirectionLocally trueDistanceMost accurate in the middle latitudes
7 Robinson – world projection Compromise projection used for world mapsShapeDistortion is very low within 45° of the origin and along the equatorAreasDistortion is very low within 45° of the origin and along the equator.DirectionGenerally distortedDistanceScale is constant along any given latitude and for latitude of the opposite signLimitationsUseful only for world maps
8 State PlaneNot 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 ShapeAccurate representation of small shapesAreasMinimal distortion within each UTM zoneDirectionLocal angles are trueDistanceScale is constant along the central meridianLimitationsError 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-DDatum transformations are from one datum to another, 3-D to 3-DChanging 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 spheroidNAD 1983 using the Geodetic Reference system (GRS) 1980 spheroidWorld Geodetic System (WGS) 1984 using the WGS 1984 spheroidInput geographiccoordinate systemNAD 1927Output geographiccoordinate systemWGS 1984
13 DatumThe 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 dataGeoreferencing 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 imagesScanned map datasets don't normally contain spatial reference informationaerial 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 systemCoordinates in the target system