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Using Geographic Information Systems (GIS)

Map Datum

Some common datum, or Global Coordinate Systems (GCS), used in North America include:

  • North American Datum of 1927 (NAD27): Local datum well suited to the United States, Canada, Mexico, and the Caribbean. Uses the Clarke 1866 spheroid.
  • North American Datum of 1983 (NAD83): An earth-centered datum that corrects NAD27 coordinates based on both earth and satellite measurements. Uses the GRS 1980 spheroid. Coordinates are very like WGS84 coordinates and can be used interchangeably with them.
  • World Geodetic System of 1984 (WGS84): Earth-centered datum common for datasets with a global extent. Uses the WGS 1984 spheroid. This is the datum that GPS coordinates are based on.

Geographic transformations between different Datum

ArcGIS gives us a warning if we attempt to add data to our map that have a different GCS, or datum. For example, if we have one layer depicting the 40 fire behavior fuel models. As with projection on-the-fly, the data frame’s GCS defaults to that of the first layer added to the map, which is North American 1983. If we then try to add a fire perimeter shapefile with the WGS 1984 geographic coordinate system, we get a warning that a geographic transformation may be necessary.

A geographic transformation, sometimes referred to as a datum transformation, is a set of mathematical formulas for converting coordinates from one datum to another. At this point, you may specify the transformation by clicking the transformations box in the warning dialog box. In most cases, the transformation at the top of the list will be the best choice.

Map Projections & Coordinate Systems

A projected coordinate system (PCS) can reference the same geographic locations using a Cartesian system, which includes a uniform, linear unit of measure.

  • Universal Transverse Mercator (UTM) divides the earth into 60 zones, each six degrees of latitude wide. Figure 9 below depicts a simplified view of the UTM zones covering conterminous United States.
UTM Boundaries for the US

  • State Plane Coordinate Systems are a good example of a PCS being independent of its map projection. Lambert Conformal Conic projections are used for domains with greater east-west extent, Transverse Mercator projections are used for domains with greater north-south extent. Some use an oblique Mercator projection.
Geographic Projections
  • LANDFIRE (http://www.landfire.gov ) uses the Albers Equal Area Conic projection for national level data products because it is well suited for data with an east-west orientation at middle latitudes, such as the continental United States. Furthermore, because this is an equal area projection, all areas on the map are proportional to the same areas on the earth.
  • WFDSS Custom Albers Projection is an Albers Equal Area Conic projection that is defined based upon the WFDSS incident spatial domain. In that sense, each incident and its associated fire behavior analyses will have a unique WFDSS Custom Albers projection, By using a custom projection for each incident, centered on that incident, reduces the distortion inherent in all projections in the area around the fire. Among other factors, the custom projection reduces the potential for errors in direction (wind direction, aspect, etc.) that could otherwise be introduced if a non-custom projection were used.

Within the WFDSS incident, landscape (lcp) files created for fire behavior analyses are stored in its Custom Albers projection and can only be downloaded in that projection. Other fire behavior outputs can be downloaded in either the WFDSS Custom Albers projection or in a geographic coordinate system.

The WFDSS Custom Albers projection is recognized by ArcGIS and datasets stored in a WFDSS Custom Albers projection can be used in ArcMap like any other dataset.

Re-projecting Shapefile or arcgrid in ArcGIS

If a shapefile or ascii grid will not display as an overlay on a landscape (lcp) in FARSITE or FLAMMAP, it cannot be used by those systems. It is most likely using a different coordinate system than the lcp does.

In this case, the file (feature/shapefile or raster/ascii grid) can be re-projected to the same coordinate system so it can be displayed onscreen and used in reference by the landscape editor in FARSITE.

ArcToolbox Menu - Data Management
  1. Open a new ArcMap window and add the shapefile or raster file that is stored in the desired projection. By adding the shapefile (or grid) with the desired projection first, the coordinate system of the Data Frame will default to the desired projection.
  2. Next, add the shapefile that is stored in the other projection.
  3. If the ArcToolbox window is not already displayed, click on the ArcToolbox icon to show the ArcToolbox window.
  4. In the ArcToolbox window, click on the plus sign next to “Data Management Tools” to expand the selection. Next, click on the plus sign next to “Projections and Transformations” to expand the selection. Next, click on the plus sign next to “Feature” (for shapefiles or “Raster” (for grids) to expand the selection.  Double-click on “Project” to open the tool.
  5. In the Project window, under “Input Dataset or Feature Class,” select the shapefile/raster grid that is currently stored with the wrong projection. The Input Coordinate System should automatically default to its projection.” If none is displayed, that means that there is no prj file accompanying it. If the projection is known, it can be specified here.
  6. Specify an output shapefile or raster grid under “Output Dataset or Feature Class.” Click on the  button next to “Output Coordinate System.” In the “Spatial Reference Properties” window that pops up, click on the “Import” button.  Navigate to and select the shapefile that is stored in desired coordinate system.  The new projection properties will load into the “Spatial Reference Properties” window.  Click “OK” on the “Spatial Reference Properties” window. 
  7. Click “OK” in the “Project” window to create a new shapefile that is stored in the chosen UTM projection.

If the user discovers the feature or raster does not have a defined projection; one can be added by selecting “Define Projection”, found under “Projections and Transformations”.

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