.. only:: html
|LS| Working with Raster Data
===============================================================================
Raster data is quite different from vector data. Vector data has discrete
features constructed out of vertices, and perhaps connected with lines and/or
areas. Raster data, however, is like any image. Although it may portray various
properties of objects in the real world, these objects don't exist as separate
objects; rather, they are represented using pixels of various different color
values.
During this module you're going to use raster data to supplement your existing
GIS analysis.
**The goal for this lesson:** To learn how to work with raster data in the QGIS
environment.
|basic| |FA| Loading Raster Data
-------------------------------------------------------------------------------
Raster data can be loaded with the same methods we used for vector data.
However we suggest to use the :guilabel:`Browser` Panel.
#. Open the :guilabel:`Browser` Panel and expand the :file:`exercise_data/raster`
folder.
#. Load all the data in this folder:
* :file:`3320C_2010_314_RGB_LATLNG.tif`
* :file:`3320D_2010_315_RGB_LATLNG.tif`
* :file:`3420B_2010_328_RGB_LATLNG.tif`
* :file:`3420C_2010_327_RGB_LATLNG.tif`
You should see the following map:
.. image:: img/raster_step_one.png
:align: center
There we have it - four aerial photographs covering our whole study area.
.. _tm_virtual_raster:
|basic| |FA| Create a Virtual Raster
-------------------------------------------------------------------------------
Now as you can see from this, your solution layer lies across all four
photographs. What this means is that you're going to have to work with four
rasters all the time. That's not ideal; it would be better to have one file for
one (composite) image, right?
Luckily, QGIS allows you to do exactly this, and without needing to actually
create a new raster file, which could take up a lot of space. Instead, you can
create a **Virtual Raster**. This is also often called a *Catalog*, which
explains its function. It's not really a new raster. Rather, it's a way to
organize your existing rasters into one catalog: one file for easy access.
To make a catalog we will use the :menuselection:`Processing --> Toolbox`.
#. Open the :guilabel:`Build virtual raster` algorithm from the
:menuselection:`GDAL --> Raster miscellaneous`;
#. In the dialog that appears click on the :guilabel:`...` button next to the
:guilabel:`Input layers` parameter and check all the layers or use the
:guilabel:`Select All` button;
#. Uncheck the :guilabel:`Place each input file into a separate band` parameter-
Notice the text field below. What this dialog is actually doing is that it's
writing that text for you. It's a long command that QGIS is going to run.
.. note:: Keep in mind that you can copy and paste the text in the
``OSGeo Shell`` (Windows user) or ``Terminal`` (Linux and OSX users) to run
the command. You can also create a script for each GDAL command. This is very
handy when the procedure is taking a long time or when you want to schedule
specific tasks. Use the :guilabel:`Help` button to get more help on the
syntax of GDAL commands.
#. Finally click on :guilabel:`Run`.
.. note:: As you know from the previous modules, :guilabel:`Processing` creates
temporary layers by default. To save the file click on the :guilabel:`...`
button.
.. image:: img/build_virtual_raster.png
:align: center
You can now remove the original four rasters from the :guilabel:`Layers` Panel
and leave only the output virtual catalog raster.
|hard| Transforming Raster Data
-------------------------------------------------------------------------------
The above methods allow you to virtually merge datasets using a catalog, and to
reproject them "on the fly". However, if you are setting up data that you'll be
using for quite a while, it may be more efficient to create new rasters that
are already merged and reprojected. This improves performance while using the
rasters in a map, but it may take some time to set up initially.
Reprojecting rasters
...............................................................................
Open :guilabel:`Warp (reproject)` from
:menuselection:`GDAL --> Raster projections`.
You can also reproject virtual rasters (catalogs), enable multithreaded
processing, and more.
.. image:: img/warp_rasters.png
:align: center
Merging rasters
...............................................................................
If you need to create a new raster layer and save it to disk you can use the
merge algorithm.
.. note:: Depending on how many raster files you are merging and their resolution,
the new raster file created can be really big. Consider instead to create
a raster catalog as described in the
:ref:`Create a Virtual Raster ` section.
#. Click on the :guilabel:`Merge` algorithm from the
:menuselection:`GDAL --> Raster miscellaneous` menu.
#. As we did for the :ref:`Create a Virtual raster `, use
the :guilabel:`...` button to choose which layers you want to merge.
You can also specify a Virtual raster as input, and then all of the rasters
that it consists of will be processed.
#. If you know the GDAL library, you can also add your own options by opening the
:guilabel:`Advanced parameters` menu.
.. image:: img/merge_rasters.png
:align: center
|IC|
-------------------------------------------------------------------------------
QGIS makes it easy to include raster data into your existing projects.
|WN|
-------------------------------------------------------------------------------
Next, we'll use raster data that isn't aerial imagery, and see how
symbolization is useful in the case of rasters as well.
.. Substitutions definitions - AVOID EDITING PAST THIS LINE
This will be automatically updated by the find_set_subst.py script.
If you need to create a new substitution manually,
please add it also to the substitutions.txt file in the
source folder.
.. |FA| replace:: Follow Along:
.. |IC| replace:: In Conclusion
.. |LS| replace:: Lesson:
.. |WN| replace:: What's Next?
.. |basic| image:: /static/global/basic.png
.. |hard| image:: /static/global/hard.png