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

    • Planetocentric

    • Positive East Longitudes (stored in 0-360 degrees but sometimes shown in -180..180 degrees depending on the projection)

  • Common center calculation

    • For lines this approximately halfway along the line from start to finish

    • For 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)

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:

  • globes (ala Google Earth/Mars/Moon) with geographic coordinates

  • 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 the shapefile with geographic latitude [-90, 90] and longitude [-180, 180] 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

    • with geographic coordinates in the range of [-90, 90] latitude and [-180, 180] longitude

    • with geographic coordinates in the range of [-90, 90] latitude and [0, 360] longitude

  • 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

    • with geographic coordinates in the range of [-90, 90] latitude and [-180, 180] longitude

    • with geographic coordinates in the range of [-90, 90] latitude and [0, 360] longitude

    • with Stereographic projection centered at the North Pole

    • with Stereographic projection centered at the South Pole

The 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 with geographic coordinates in the range of [-90, 90] latitude and [0, 360] longitude

Holds an individual footprint with geographic coordinates in the range of [-90, 90] latitude and [0, 360] longitude. Can be found under the Derived File tab of the product’s results page (assuming the product has location data).

Individual Product Shapefile Zip or Tar.Gz File with geographic coordinates in the range of [-90, 90] latitude and [-180, 180] longitude

Holds an individual footprint with geographic coordinates suitable for any projection. 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)

Holds the footprints of all individual product footprints of an ODE mission / instrument / product type. Please see Footprint Coverage Maps help

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.