Introduction to ArcGIS forEnvironmental ScientistsModule 2 – GIS FundamentalsLecture 5 – Coordinate Systems and Map Projections
Chapter 5 Objectives • Terms – spheroids, datums, coordinate systems, and projections • Types of projections • Projections in ArcGIS
Overview • GIS must accurately represent locations of features from the earth’s surface on a map • This requires a reference system, or coordinate system
Definitions I • Coordinate System (CS) – provides a frame of reference to define locations • Geographic CS – Used for 3D (sphere or globe), locations defined by latitude and longitude, usually in decimal degrees • Projected CS – Used for 2D (maps), locations defined by x,y measured from some origin (0,0), usually in meters, sometimes in feet
Definitions II • Spheroid – A simplified model of the shape of the earth • Spheroid is used interchangeably with ellipsoid • Datum – Defines the spheroid being used and aligns the spheroid, optimizing fit either for a specific area, or overall globally. This also defines the origin point of the coordinate system.
Definitions III • Projection – A mathematical equation to convert from spherical (3-D) to planar (2-D) coordinates • Every projection will cause some form of spatial distortion in shape, area, distance, and/or direction • Common projections used in the US: • Albers Equal Area for national scale • State Plane and UTM for state and local scales
North American Datums • Common North American datums are: • NAD27, Clarke 1866 spheroid and Meade’s Ranch, KS as the origin point • NAD83, GRS80 (GRS = Geodetic Reference System) spheroid and the earth’s center of mass as it’s origin • WGS84, WGS84 spheroid and the earth’s center of mass as it’s origin. This datum is used globally • In North America, WGS84 is virtually identical to NAD83 • Other datum changes will change coordinates of spatial features (e.g. NAD83 to NAD27)
Types of Projections • Imagine a globe with a light source inside that projects features on the earth’s surface onto a flat surface • That flat surface can be configured a number of ways – as a cylinder, a cone, or a plane
Projections in ArcGIS • All geographic datasets have a GCS • Many also have a PCS • Unprojected (GCS) data is generally inappropriate for GIS analyses • The first layer added to ArcMap defines the projection and datum for the data frame. This can be changed in the data frame properties. • All subsequent layers are projected on-the-fly (if necessary) to match the data frame projection. The GCS of the layer must match the data frame or a transformation is required.
Projection Metadata • Stored in: • A .prj file for shapefiles • A table in geodatabases • A prj.adf in coverages and grids • A header for other raster formats. This can be either a separate file (.hdr, .tifw) or embedded in the raster file itself • Data have a GCS and may have a PCS even if the metadata is missing. These data have an unknown coordinate system.
Projection Metadata • CS information can be viewed in ArcMap (layer properties>source tab) and ArcCatalog (Description tab)
Data with Unknown Coordinate Systems • Cannot be projected on-the-fly • Will generate a warning when added to ArcMap • Layer will be added, but may not align with other data • Can be fixed with Define Projection tool if you know the CS • If you don’t, you can try: • contacting the source • adding the layer to a map with a known CS. If the new layer is registered with existing layers, it is the same CS.
Projecting Raster Data • Because projections cause distortions in size and shape, and raster data must have square cells, cells in projected rasters represent different areas on the surface of the earth • Resampling is used to assign values to projected cells. • Nearest neighbor is fastest and must be used for categorical data like land cover • Bilinear interpolation uses a weighted distance average of surrounding cells. Smooths values moderately, useful for continuous data like elevation • Cubic convolution fits a curve through surrounding points. Can smooth data significantly and is slower
Map Units vs. Display Units • Coordinates of the dataset are stored in map units. Geographic generally uses decimal degrees, while a PCS will usually use meters or feet. Map units can only be changed by changing the CS of the data (Project tool). • Display units are independent of map units and are set in the data frame properties. ArcMap reports coordinate values and measurements in display units.
Points to Remember • Choose a PCS for each project. When new data is acquired, be sure it is in that PCS or convert it using the Project tool. It is best not to rely on projection on-the-fly in ArcMap. It is especially important to have all data in the same PCS when doing analyses. • Try and avoid changing the CS of raster data • Use the Project tool to change the CS of data, but use the Define Projection tool to add metadata about the CS • CS should be defined for all spatial data
More information • http://nationalatlas.gov/articles/mapping/a_projections.html • Good basic information • http://egsc.usgs.gov/isb/pubs/MapProjections/projections.html • Excellent source provides characteristics and scale suitability for a dozen or so common CSs • On-line ESRI Virtual Campus course - Understanding Map Projections and Coordinate Systems