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What are product footprints and how do I use them?

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Many geosciences-related products have a surface location.  This surface location can be described with a “footprint”, i.e. the area of the surface that is covered by the product.  The specifics of the footprint, i.e. how the product “covers” the surface, will vary depending on the product type.  Image products have a coverage area, i.e. the area covered by the image.  But products like the SHARAD instrument have a coverage better described as a line across the surface.  Other products like the MOLA PEDR records are individual measurements at a given lat/lon, which are best described by points.  

 

ODE uses product coverage footprints aid in finding products by using locations and helping the user determine if the product meets their needs.  To support cross-mission/instrument searching and visualization, ODE generates footprints in common coordinate systems with common attributes such as how the footprint center is determined.  Footprints are generated in a variety of methods: in some cases, the footprint is directly taken from the label (footprint or reticule points or lat/lon bounding boxes); from the appropriate PDS Archive index file; directly from the product or related products (example: CRISM TRDR from the DDR data file); or from USGS’s Unified Planetary Coordinate database.  The specific method used for a given product can be found in the ODE notes at the bottom of the product result page’s metadata tab page.

 

A few highlights about footprints:

Common coordinate system
oPlanetocentric
oPositive East Longitudes (stored in 0-360 degrees but sometimes shown in -180..180 degrees depending on the projection)
Common center calculation
oFor lines this approximately halfway along the line from start to finish
oFor areas, the center is usually calculated as the intercept between two great circle routes – one from the max lat/west longitude corner to the min lat/east longitude corner and one from the min lat/east longitude corner to the max lat/west longitude corner.  This approach generally works well for footprints that range from simple rectangles to long image strips typical of many imaging instruments.  However, it can break down at the poles and place the center along the edge of the footprint or even, in some cases, outside the footprint.
Points/vertices are rounded to three decimal places
A single product may have multiple individual footprints depending on the nature of the instrument and the noise in the data.
Complex area footprints that cross over themselves have been broken into multiple simple footprints that do not cross over themselves (typically due to noisy data)

 

ComplexFootprints

Figure 5 - Complex Footprints to Simple Footprints

 

Footprint Type

Attributes

Area

Footprint Outline

Lat/Lon Bounding Box

Center Point – location at approximately the center of the polygon

Line

Footprint Surface Line

Lat/Lon Bounding Box

Center Point – located along the surface line halfway from the start to the end

Point

Center Point

 

ODE currently offers three ways to use footprint data:

through ODE’s own web map interface
via the Google Earth/Mars/Moon globe tool;
within GIS tools that read ESRI’s Shapefile format including ESRI’s ArcGIS tools.

 

Footprint data is often used in a variety of map projections:

cylindrical projections with a center longitude of zero (longitude range of -180 to 180)
cylindrical projections with a center longitude of 180 (longitude range of 0 to 360)
globes (ala Google Earth/Mars/Moon)
polar stereographic projections

 

To meet the various interface and projection needs, ODE generates a wide range of footprint coverage data in a variety of formats.  ODE itself uses cylindrical center longitude zero projected footprints in its own map display.  ODE also generates:

Individual footprints in KML global projections for Google Earth/Mars/Moon
Individual footprints in ESRI Shapefile format for Shapefile enabled GIS tools
oCylindrical Center Longitude Zero Projection
oCylindrical Center Longitude 180 Projection
oGlobal Projection (suitable for Polar Stereographic Projections)
Product type coverage maps in KMZ global projections for Google Earth/Mars/Moon
Product type coverage maps in ESRI Shapefile format for Shapefile enabled GIS tools
oCylindrical Center Longitude Zero Projection
oCylindrical Center Longitude 180 Projection
oGlobal Projection (suitable for Polar Stereographic Projections)
oGlobal North Projection
oGlobal South Projection
oAssociated Centers in Global Projection

 

The cylindrical projected footprints are modified to handle edge crossings (by splitting footprints that cross the left and right edge) and pole conditions (by adding vertices at the poles).

 

Individual Product Derived Footprint File Types


Individual Product KML/ KMZGlobal Footprint File

Holds an individual footprint.  All longitudes are in the form -179.999 to 180.  The footprints also include a center point.  Can be found under the Derived File tab of the product’s results page (assuming the product has location data).

Individual Product Global Shapefile Zip or Tar.Gz File

Holds an individual footprint suitable for polar projections.  All longitudes are in the range 0..360.  Can be found under the Derived File tab of the product’s results page (assuming the product has location data).

Individual Product Cylindrical 0..360 (Center longitude 180) Shapefile Zip or Tar.Gz File

Holds an individual footprint suitable for cylindrical projections with a center longitude of 180.  All longitude are in the range 0..360.  Can be found under the Derived File tab of the product’s results page (assuming the product has location data).

Individual Product Cylindrical -180..180 (Center longitude 0) Shapefile Zip or Tar.Gz File

Holds an individual footprint suitable for cylindrical projections with a center longitude of 0.  All longitude are in the range -180..180.  Can be found under the Derived File tab of the product’s results page (assuming the product has location data).

Footprint Coverage Maps (KMZ, Shapefiles)

Please see Footprint Coverage Explorer Page

 

Shapefiles are actually made up of five individual files: .SHP, .SHX, .PRJ, .SHP.XML, and .DBF.  To aid in downloading the entire shapefile, the individual shapefile files are provided together in a zip or tar.gz compressed file.