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<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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   [<!ENTITY appname          "Viking">]
>

<section id="GeoRef" xreflabel="GeoRef"><title>GeoRef Layer</title>
<para>
This layer allows one to use any image as a map. You will need to tell it how to align the map to a position in the world and the scale of map.
</para>
<para>
When a new image is selected the values from a <ulink url="https://en.wikipedia.org/wiki/World_file">World File</ulink> will automatically be tried to be found. 
(subject to the setting in <xref linkend="prefs_external"/>)
</para>
<para>
If you do not know the scale of the map but know the coordinates of both upper left and lower right coordinates,
 you can enter these coordinates (Lat/Lon only) and then use the <guibutton>Calculate MPP values from coordinates</guibutton> button.
</para>
<para>
The map once loaded can be repositioned via <xref linkend="georef_tools"/>, which may be easier than trying to edit the raw property values.
</para>

<section><title>Layer Properties</title>
<para>
Configurable properties:
</para>
<itemizedlist>
<listitem>
	<para>The map image file.</para>
</listitem>
<listitem>
	<para>X + Y Scales in metres per pixel.</para>
</listitem>
<listitem>
	<para>Rotation (in degrees), clockwise about the upper left corner position.</para>
</listitem>
<listitem>
	<para>Coordinates of the upper left corner of the map (either UTM or Lat/Lon value).</para>
</listitem>
<listitem>
	<para>Alpha. Control the Alpha value for transparency effects using a value between 0 and 255 with the default being 255 for a fully solid image, whereas 0 is transparent.</para>
</listitem>
</itemizedlist>
<figure>
<title>Properties</title>
<screenshot>
  <graphic format="PNG" fileref="figures/GeoRefLayer_Properties.png"/>
</screenshot>
</figure>
<section><title>World File Notes</title>
<para>
  &appname; does not support "upside down" images, nor does it support the +ve/-ve definitions of the Y-Scale.
  Instead the Y-Scale is treated as in the direction of the image, thus reading in from a World File the +ve/-ve of the Y-Scale is ignored
  and -ve values can not input directly into the GUI (simply use the absolute value instead).
</para>
<para>
  There is limited support for reading in skew values.
  As &appname; uses a single rotation value, this will be derived from the y-skew value and the x-skew is ignored.
</para>
</section>
</section><!-- Prop END -->

<section><title>Layer Operations</title>
<section><title>Zoom to Fit Map</title>
<para>
Automatically set a zoom level to see the entire map coverage.
</para>
</section>

<section><title>Goto Map Center</title>
<para>
Center the viewport on this map.
</para>

</section>
<section><title>Export to World File</title>
<para>
Opens a file save dialog to save the scale and position information as a <ulink url="https://en.wikipedia.org/wiki/World_file">World File</ulink>.
</para>
</section>
</section>

</section><!-- End GeoRef -->
