Raster maps: GeoTIFF format

The GeoTIFF raster format is a type of TIFF image file with embedded information that documents how that image is associated with the real world ("georeferencing information").

It is a standard format that is used to provide imagery that can be imported into GIS applications so that they are placed properly relative to other features in the world.

Xastir supports reading a limited set of such images.

Requirements for using GeoTIFF files

The GeoTIFF feature requires that you have the libgeotiff library installed. This library has its own prerequisites, including the TIFF library and the PROJ projection library. See Installing Xastir. Most operating systems have this package in their package management systems, and installing that package should pull in all its prerequisites.

Using GeoTIFF maps

If your GeoTIFF file is one of the types Xastir can read, just put it in one of the directories Xastir searches for maps (any subdirectory of /usr/local/share/xastir/maps, for example) and reindex your maps. You can then choose it in the map chooser.

GeoTIFF is NOT limited to USGS Topo Maps

The GeoTIFF map format was originally added to Xastir to support direct import of USGS Topographic Quadrangles ("Topo Quads") as they were created by the United States Geological Survey, and in many ways is optimized for use with those maps. But contrary to what the configure script and some of our older documentation states, the feature is not strictly limited to those types of data.

Xastir's ability to snip off the collars of maps and stitch them seamlessly is very closely tied to maps in UTM.

Xastir's GeoTIFF support is limited to UTM and Geographic coordinate systems

Xastir is not a full-featured GIS, and its use of GeoTIFF files makes a number of assumptions that make it unsuitable for use with all GeoTIFF files.

Each GeoTIFF file has associated with it the map projection in which it was made.

Xastir will properly display and georeference any GeoTIFF file that is in the Universal Transverse Mercator projection (UTM) or in WGS84 Geographic Coordinate system (EPSG:4326).

Maps in other projections might read into Xastir and get displayed, but if they are they will be distorted or improperly placed.

Using USGS Topo Maps in GeoTIFF format

These maps come with a GeoTIFF file and a separate "FGD" file. Put both in the same directory, and Xastir will use the information in the FGD file to locate the pixels in the image that are inside the "neatline" of the map, and discard everything outside that. This enables those maps to be stitched together seamlessly.

Using maps that are in UTM but not USGS Topo Maps

They'll just work, but making them seamlessly tile together is more work.

Creating a "fake" FGD file

If your map is in UTM and you can identify the latitude and longitude of the edges of the real part of the map (the "neatline"), you can construct a "fake" FGD file for them. Give it the same base name as the GeoTIFF file, but with a ".fgd" extension (e.g. O34106H3.tif and O34106H3.fgd).

The format of this fake FGD file should be:

West_Bounding_Coordinate: <longitude of the west edge of map in decimal degrees>
East_Bounding_Coordinate: <longitude of the east edge of map in decimal degrees>
North_Bounding_Coordinate: <latitude of the north edge of map in decimal degrees>
South_Bounding_Coordinate: <latitude of the south edge of map in decimal degrees>

Note that you have to get the signs of latitude and longitude right. Longitudes west of the prime meridian are negative numbers, latitudes south of the equator are negative.

Using maps in WGS84 geographic coordinate system

These maps will import into Xastir just fine, but making them stitch seamlessly isn't straightforward. IF the neatlines in the image are straight lines that are parallel to the edges of the image, you can do it with a fake FGD file just as above.

If they are curved, well, you can't do it with an FGD file. This is an advanced map topic.

How can I tell if Xastir can use this GeoTIFF?

You need to determine what projection was used for the map. This is a fairly simple thing to determine. Xastir can use any map in UTM or WGS84 Geographic projection without modification.

How to tell what projection your GeoTIFF file is in

When you installed the "libgeotiff" library and its headers, you should also have gotten a tool called "listgeo". If not, you might have to install a second package (on Ubuntu, for example, the package "geotiff-bin"). This tool can read your GeoTIFF file and list the information in its metadata.

example: the Escabosa, NM quadrangle map, O34106H3.tif

> listgeo  O34106H3.tif
Geotiff_Information:
   Version: 1
   Key_Revision: 0.2
   Tagged_Information:
      ModelTiepointTag (2,3):
         0                 0                 0
         373316.521046     3874858.551806    0
      ModelPixelScaleTag (1,3):
         2.4384            2.4384            0
      End_Of_Tags.
   Keyed_Information:
      GTModelTypeGeoKey (Short,1): ModelTypeProjected
      GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
      ProjectedCRSGeoKey (Short,1): Code-26913 (NAD83 / UTM zone 13N)
      ProjectedCitationGeoKey (Ascii,25): "UTM Zone 13 N with NAD83"
      End_Of_Keys.
   End_Of_Geotiff.

PCS = 26913 (NAD83 / UTM zone 13N)
Projection = 16013 (UTM zone 13N)
Projection Method: CT_TransverseMercator
   ProjNatOriginLatGeoKey: 0.000000 (  0d 0' 0.00"N)
   ProjNatOriginLongGeoKey: -105.000000 (105d 0' 0.00"W)
   ProjScaleAtNatOriginGeoKey: 0.999600
   ProjFalseEastingGeoKey: 500000.000000 m
   ProjFalseNorthingGeoKey: 0.000000 m
GCS: 4269/NAD83
Datum: 6269/North American Datum 1983
Ellipsoid: 7019/GRS 1980 (6378137.00,6356752.31)
Prime Meridian: 8901/Greenwich (0.000000/  0d 0' 0.00"E)
Projection Linear Units: 9001/metre (1.000000m)

The thing we're looking for here is that the PCS ("Projected Coordinate System") is UTM. This file will be happily read in by Xastir and display properly, in the correct spot in the world.

The other form of GeoTIFF we can read are those in WGS84 Latitude/Longitude ("Geographic coordinate system").

An example of a GeoTIFF in geographic coordinates

> listgeo Albuquerque_75_North_ll.tif
Geotiff_Information:
   Version: 1
   Key_Revision: 1.0
   Tagged_Information:
      ModelTiepointTag (2,3):
         0                 0                 0
         -110.216970855117 36.2799546362302  0
      ModelPixelScaleTag (1,3):
         0.000686415531838486 0.000686415531838486 0
      End_Of_Tags.
   Keyed_Information:
      GTModelTypeGeoKey (Short,1): ModelTypeGeographic
      GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
      GeographicTypeGeoKey (Short,1): GCS_WGS_84
      GeogCitationGeoKey (Ascii,7): "WGS 84"
      GeogAngularUnitsGeoKey (Short,1): Angular_Degree
      End_Of_Keys.
   End_Of_Geotiff.

GCS: 4326/WGS 84
Datum: 6326/World Geodetic System 1984
Ellipsoid: 7030/WGS 84 (6378137.00,6356752.31)
Prime Meridian: 8901/Greenwich (0.000000/  0d 0' 0.00"E)
Projection Linear Units: User-Defined (1.000000m)

Here, we see that this map is in Geographic Coordinate system ("GCS") and WGS84 datum. This file is also going to display well in Xastir.