.. _gdal_viewshed:

================================================================================
gdal_viewshed
================================================================================

.. only:: html

    .. versionadded:: 3.1.0

    Calculates a viewshed raster from an input raster DEM using method defined in [Wang2000]_ for a user defined point.

.. Index:: gdal_viewshed

Synopsis
--------

.. code-block::

   gdal_viewshed [-b <band>]
                 [-a_nodata <value>] [-f <formatname>]
                 [-oz <observer_height>] [-tz <target_height>] [-md <max_distance>]
                 -ox <observer_x> -oy <observer_y>
                 [-vv <visibility>] [-iv <invisibility>]
                 [-ov <out_of_range>] [-cc <curvature_coef>]
                 [[-co NAME=VALUE] ...]
                 [-q] [-om <output mode>]
                 <src_filename> <dst_filename>

Description
-----------

By default the :program:`gdal_viewshed` generates a binary visibility raster from one band
of the input raster elevation model (DEM). The output raster will be of type
Byte. With the -mode flag can also return a minimum visible height raster of type Float64.

.. note::
    The algorithm as implemented currently will only output meaningful results
    if the georeferencing is in a projected coordinate reference system.

.. program:: gdal_viewshed

.. include:: options/co.rst

.. option:: -b <band>

   Select an input band **band** containing the DEM data. Bands are numbered from 1.
   Only a single band can be used. Only the part of the raster within the specified 
   maximum distance around the observer point is processed.

.. option:: -a_nodata <value>

   The value to be set for the cells in the output raster that have no data.

   .. note::
        Currently, no special processing of input cells at a nodata
        value is done (which may result in erroneous results).

.. option:: -ox <value>

   The X position of the observer (in SRS units).

.. option:: -oy <value>

   The Y position of the observer (in SRS units).

.. option:: -oz <value>

   The height of the observer above the DEM surface in the height unit of the DEM. Default: 2

.. option:: -tz <value>

   The height of the target above the DEM surface in the height unit of the DEM. Default: 0

.. option:: -md <value>

   Maximum distance from observer to compute visibiliy.
   It is also used to clamp the extent of the output raster.

.. option:: -cc <value>

   Coefficient to consider the effect of the curvature and refraction.
   When calculating visibility between two points (i.e. Line Of Sight or Viewshed),
   The magnitude of this effect varies with atmospheric conditions and depends on the wavelength.

   Different applications for calculating visibility use different interchangeable notation to describe this phenomena:
   Refraction Coefficient, Curvature Coefficient, and Sphere Diameter Factor.
   gdal_viewshed uses the Curvature Coefficient notation.

   .. math::

     {CurvCoeff}=1-{RefractionCoeff}

   Changes in air density curve the light downward causing an observer to see further and the earth to appear less curved,
   as if the sphere (earth) diameter is larger then it actually is.
   The ratio between that imaginary sphere diameter and the actual sphere diameter is given by the formula:

   .. math::
     {SphereDiameterFactor}=1/{CurvCoeff}=1/(1-{RefractionCoeff})

   For visible light, the standard atmospheric refraction coefficient that is generally used is 1/7.
   Thus the default value for CurvCoeff that gdal_viewshed uses is 0.85714 (=~ 1-1/7).

   The height of the DEM is corrected according to the following formula:

   .. math::

      Height_{Corrected}=Height_{DEM}-{CurvCoeff}\frac{{TargetDistance}^2}{SphereDiameter}

   Typical coefficient values are given in the table below (use Curvature Coeff value for the cc option)

   ================  ==================  ===================  =====================
   Use Case          Refraction Coeff    **Curvature Coeff**  Sphere Diameter Factor
   No Refraction     0                   1                    1
   Visible Light     1/7                 6/7 (=~0.85714)      7/6 (=~1.1666)
   Radio Waves       0.25 ~ 0.325        0.75 ~ 0.675         1.33 ~ 1.48
   Flat Earth        1                   0                    inf
   ================  ==================  ===================  =====================

.. option:: -iv <value>

   Pixel value to set for invisible areas. Default: 0

.. option:: -ov <value>

   Pixel value to set for the cells that fall outside of the range specified by
   the observer location and the maximum distance. Default: 0

.. option:: -vv <value>

   Pixel value to set for visible areas. Default: 255

.. option:: -om <output mode>

  Sets what information the output contains.

  Possible values: VISIBLE, DEM, GROUND

  VISIBLE returns a raster of type Byte containing visible locations.

  DEM and GROUND will return a raster of type Float64 containing the minimum target
  height for target to be visible from the DEM surface or ground level respectively.
  Flags -tz, -iv and -vv will be ignored.

  Default VISIBLE

C API
-----

Functionality of this utility can be done from C with :cpp:func:`GDALViewshedGenerate`.

Example
-------

Compute the visibility of an elevation raster data source with defaults


.. figure:: ../../images/gdal_viewshed.png

   A computed visibility for two separate `-ox` and `-oy` points on a DEM.

.. code-block::

    gdal_viewshed -md 500 -ox -10147017 -oy 5108065 source.tif destination.tif




.. [Wang2000] Generating Viewsheds without Using Sightlines. Wang, Jianjun,
   Robinson, Gary J., and White, Kevin. Photogrammetric Engineering and Remote
   Sensing. p81. https://www.asprs.org/wp-content/uploads/pers/2000journal/january/2000_jan_87-90.pdf
