In the tutorial Georeferencing Topo Sheets and Scanned Maps (QGIS3) we covered the basic process of georeferencing in QGIS. That method involved reading the coordinates from your scanned map and entering them manually as control points. Many times though you may not have the coordinates printed on your map, or you are trying to georeference an image. In that case, you can use another georeferenced data-source as your input. In this tutorial, you will learn how to use existing open data sources in your georeferencing process.
We will georeference high resolution balloon-imagery using reference coordinates from OpenStreetMap.
In this tutorial, we will be using kite and balloon imagery collected by The Public Laboratory. They make the georeferenced versions of the images also available, but we will download a non-georeferenced JPG image and go through the process of georeferencing it in QGIS.
Download the JPG image of Washington Square Park, New York. You can right-click the JPG button and choose Save link as....
For convenience, you may directly download a copy of the dataset from the link below:
OpenStreetMap
layer under XYZ Tiles in the Browser Panel. Drag the layer to the main canvas. Once loaded, note the Coordinate Reference System (CRS) for this layer in the bottom-right corder. It is set as EPSG 3857 Pseudo Mercator
. This is important because the coordinates we infer from this layer during georeferencing will be in this CRS.Note
See this page for more details on XYZ layers and how to add other basemaps in QGIS.
Washington Square Park, New York
. You can zoom/pan try to locate this park in the map. But that is cumbersome and may not be practical. An easier way is to use the OpenStreetMap (OSM) Place Search plugin to search for the exact location. Install the OSM Place Search
plugin from . If you do not see this plugin in the search results, make sure you have checked Also show experimental plugins under Settings. See Using Plugins for more information on using plugins in QGIS.Washington Square Park
in the Name contains.. box and click ->. You will see the matching place names appear in the results panel. Select the correct result and click the Zoom button.Polynomial 2
. See QGIS Documentation to learn about different transofrmation types and their uses. As noted earlier, our basemap is in EPSG 3857 Pseudo Mercator
CRS, so set that as the Target CRS. You can leave the Output raster name to the default and choose LZW
as the Compression. Check the Use 0 for transparency when needed. Make sure the Load in QGIS when done option is checked. CLick OK.Note
Tip: When selecting a GCP on a building, always choose the bottom of the building. Many aerial and satellite imagery have leaning buildings, so choosing a point on the rooftop will introduce errors.
dX
, dY
and Residual
error values. If a particular GCP has unusually high error values, that usually means a human-error in entering the coordinate values. So you can delete that GCP and capture it again.255
as the Additional no data value and click OK.Note
8-bit images have pixel values in the range 0-255. 0 is black and 255 is white.
This work is licensed under a Creative Commons Attribution 4.0 International License