View source for Georeferencing (Tutorial)

:''This article is part of the [[QGIS tutorial 2013/14]].<br/>In this article you learn how to georeference a raster image.''


== Step by step guide ==
:{|class='wikitable floatright'
|+ Table A 
! GCP No.
! Easting
! Northing
! Height
|-
| 01
| 566399.97
| 5712572.95
| 225.07
|-
| 04
| 566491.76
| 5712396.51
| 232.50
|-
| 05
| 566490.18
| 5712492.46
| 224.70
|-
| 06
| 566489.27
| 5712570.51
| 222.00
|-
| 08
| 566369.05
| 5712371.58
| 214.59
|-
| 09
| 566370.13
| 5712508.13
| 217.87
|}

# First, you need an image, e.g. an aerial photograph, without spatial information. For exercise purposes, you can generate one from the [[Course data|course data]] by opening the file ''aerial_utm_32_WGS84.tif'' from the ''geodata/raster/aerial/'' folder with an image processing program (e.g. Irfanview under Windows). Simply save the file under a different name, e.g. ''image_raw.tif'' and open it in [[QGIS]]. [[QGIS]] will issue a warning that the layer lacks spatial information, prompting you to select a coordinate system. You can pick one ad libitum, e.g. WGS84 (EPSG:4326) or Gauss-Kruger zone 3 (EPSG:31467), and confirm with {{button|text=OK}}. 
#* If you look at the coordinates under {{button|text=Coordinate}} below the [[QGIS GUI|canvas]] you will recognize that the coordinates are far off the real position of the map. The top left corner of the image is located at the coordinate systems point of origin (thus 0,0), which you can see by simply positioning the mouse cursor at the corner.

# In the georeferencer dialogue, click the {{button|text=Add raster}} [[Image:QGIS_2.0_georef_addrast.png|20px]] icon. Browse for the image and click through the CRS-warning dialogue as described above.
# Now you have to add some ground control points (GCP's)!
## Find the ground control points displayed in the figures below (figures '''B''' and '''C'''). The points are located at manholes, so that they can be relocated with a certain precision.
## Decide for one arbitrary point to start with, and zoom in on it using the {{button|text=zoom}} tool [[Image:QGIS_2.0_georef_zoom.png|20px]].
## When you have located the right spot, add a GCP using the {{button|text=Add point}} tool [[Image:QGIS_2.0_georef_addpoint.png|20px]].
## A menu will pop up (figure '''D'''), prompting you for the GCP coordinates. Simply copy and paste the coordinates from '''table A'''.
# Repeat this routine for all points. When you're done, open the {{mitem|text=Settings --> Transformation settings}} menu and configure the settings as follows:
#* As {{button|text=Transformation type}} select {{button|text=Helmert}}. 
#* Enter name and directory for the output file, e.g. ''<my_geodata_folder>/aerial_georef.tif'' 
#* As {{button|text=Target SRS}} (Spatial reference system), choose the WGS84/UTM zone 32N (EPSG: 32632), as this is the coordinate system used for GPS data.
#* Finally, click the {{button|text=Start georeferencing}} button [[Image:QGIS_2.0_georef_run.png|20px]].
# [[Exercise 01: First steps in QGIS|Add]] the resulting layer to the [[QGIS GUI|canvas]] and zoom in on it [[Image:QGIS_2.0_zoomtolayer.png|20px]].
# You can check the quality of the georeferenciation by comparing the georeferenced map to the actual ''aerial_utm_32_WGS84.tif'' you loaded in the beginning. You can also load the topographic map ''topo_utm32_WGS84.tif'' from the ''geodata/raster/thematic/'' directory as a reference (figure '''E''').
# To reduce the overall transformation error, you may experiment with different transformation algorithms, which can be selected in the {{mitem|text=Settings --> Transformation settings}} {{button|text=Transformation type}} menu (see above).

== Figures ==
{|class='wikitable'
|-
|[[Image:RemSens_Exercise03_6.png|thumb|450px|'''Figure A''': [[QGIS]] 2.0 with open georeferencing dialogue.]]
|[[Image:RemSens_Exercise03_4.png|thumb|450px|'''Figure B''': Ground control points (GCP's) for georeferencing.]]
|[[Image:RemSens_Exercise03_5.png|thumb|450px|'''Figure C''': Ground control points (GCP's) for georeferencing (zoomed in).]]
|}

{|class='wikitable'
|-
|[[Image:RemSens_Exercise03_7.png|thumb|450px|'''Figure D''': Coordinate prompt after adding a groud control point in the georeferencing menu.]]
|[[Image:RemSens_Exercise03_9.png|thumb|450px|'''Figure E''': Georeferenced aerial photograph with underlying topographic map.]]
|}

[[category:QGIS exercises 2013/14]]