.. _geodiff:

geodiff
-------

The ``geodiff`` program takes as input two DEMs (or a DEM and a CSV
file, with the latter in the same format as used for ``pc_align`` and
``point2dem``), and subtracts the second from the first. The grid used
is the one from the first DEM, so the second one is interpolated into it
using bilinear interpolation (when one file is a CSV, the grid from the
other one, the DEM, is used). The tool can also take the absolute
difference of the two DEMs.

It is important to note that the tool is very sensitive to the order of
the two DEMs, due to the fact that the grid comes from the first one.
Ideally the grid of the first DEM would be denser than the one of the
second.

Usage::

    geodiff [options] <dem1> <dem2> [ -o output_file_prefix ]

Example with two DEMs (when computing an absolute difference)::

    geodiff --absolute dem1.tif dem2.tif -o run
 
This will create ``run-diff.tif``.

The ``colormap`` program (:numref:`colormap`) can be used to
colorize the difference image.

Example with a DEM and a CSV file::

    geodiff dem1.tif file.csv                         \
      --csv-format '1:lon 2:lat 3:height_above_datum' \
      -o run

Command-line options for ``geodiff``:

-o, --output-prefix <filename>
    Specify the output prefix.

--absolute
    Output the absolute difference as opposed to just the difference.

--float
    Output using float (32 bit) instead of using doubles (64 bit).

--csv-format <string>
    Specify the format of input CSV files as a list of entries
    column_index:column_type (indices start from 1).  Examples:
    ``1:x 2:y 3:z`` (a Cartesian coordinate system with origin at
    planet center is assumed, with the units being in meters),
    ``5:lon 6:lat 7:radius_m`` (longitude and latitude are in degrees,
    the radius is measured in meters from planet center), 
    ``3:lat 2:lon 1:height_above_datum``,
    ``1:easting 2:northing 3:height_above_datum``
    (need to set ``--csv-proj4``; the height above datum is in
    meters).  Can also use radius_km for column_type, when it is
    again measured from planet center.

--csv-proj4 <proj string>
    The PROJ.4 string to use to interpret the entries in input CSV
    files, if those files contain Easting and Northing fields. If
    not specified, it will be borrowed from the DEM.

--nodata-value <float (default: -32768)>
    The no-data value to use, unless present in the DEM geoheaders.

--threads <integer (default: 0)>
    Select the number of threads to use for each process. If 0, use
    the value in ~/.vwrc.
 
--cache-size-mb <integer (default = 1024)>
    Set the system cache size, in MB.

--tile-size <integer (default: 256 256)>
    Image tile size used for multi-threaded processing.

--no-bigtiff
    Tell GDAL to not create bigtiffs.

--tif-compress <None|LZW|Deflate|Packbits (default: LZW)>
    TIFF compression method.

-v, --version
    Display the version of software.

-h, --help
    Display this help message.