With the release of ArcGIS Pro version 1.4 comes the ability to georeference your raster datasets using tools in the new Imagery tab. Raster users frequently have to adjust aerial imagery or scanned maps in their GIS. In this blog post, we’ll take a look at the steps required to line up your rasters to an existing dataset in a known coordinate system, so the raster data can be used in mapping and analysis.
ArcGIS Pro 1.4 Capabilities
ArcGIS Pro 1.4 brought us a wealth of new and exciting functionality to the next-gen desktop product. For some long-time users of ArcMap, there has been a bit of hesitance in adopting ArcGIS Pro. Some were concerned that specific workflows vital to their operations were not equivalent. With this new version of Pro, I think that these users are going to feel that now’s the time to jump on the Pro bandwagon. ArcGIS Pro provides the solutions you need and even expands on current capabilities; it facilitates smooth, seamless workflows through ribbons and contextual buttons. ArcGIS Pro version 1.4 brings new tools and functionalities that many GIS Analysts need in their day-to-day work. An exhaustive list of all that is new and improved in ArcGIS Pro 1.4 can be found in the ArcGIS Pro documentation. In this post, we’ll take a look at the new ArcGIS Pro georeferencing capabilities that come with the new Imagery tab in version 1.4. The ArcGIS blog post, There’s a New Tab in Town, introduced the new Imagery tab, which amalgamates common raster tools in one single ribbon interface.
The new Imagery toolbar in ArcGIS Pro 1.4.
The Georeferencing tab contains all of the tools necessary to georeference your raster datasets. This tutorial assumes you understand the basics of georeferencing and would like to become familiar with the tools available to do so in Pro. For more information on the fundamentals of georeferencing a raster dataset, I’d suggest starting with the ArcMap documentation.
The new georeferencing tools available in ArcGIS Pro 1.4.
In this post, we’ll be georeferencing a TIF image to the Imagery basemap. The image is available from the ArcTutor dataset, which can be downloaded from My Esri if you are currently on maintenance. This data is great for learning new functionality in ArcMap because it’s referenced in each of the ArcGIS tutorials. Once the ArcTutor data is installed, navigate to C:\arcgis\ArcTutor\Raster\Data\Amberg_scans\Scans to find the image 110211.tif.
So, let’s get started georeferencing in ArcGIS Pro! Launch Pro up by opening a new or existing Pro Project and add a new Map to a Project. Add a basemap as reference data for georeferencing the .tif image in this exercise; I suggest the Imagery basemap.
Preparing to Align the Image
Add in 110211.tif. In the Table of Contents, right-click the image > Zoom to Layer. The image is of a suburban area in Germany with a sports park in the southwest and a highway from the north to southeast. When I’m first looking at a new image that I need to georeference, I like to examine the different components that comprise it. For example, I compare houses versus large buildings and local roads versus highways to get a feel of what I am going to be georeferencing. I also like to look for good spots in the image that can be used as Control Points, like crisp corners of buildings.
In the Contents pane, make sure 110211.tif is selected. Along the top ribbon, choose the Imagery tab and then the Georeference button to open the Georeferencing tab.
The first section of the Georeferencing toolbar, Prepare, contains the tools to roughly place your image correctly in the map in coordination with your reference data. Click the Locate tool to search for ‘Amberg, Bayern, Deutschland’ near 11.86°E, 49.44°N; this is the general location of the scanned image.
Using the Imagery basemap, we can georeference the image in Amberg, Germany.
Use the Fit to Display tool on the Georeferencing toolbar to roughly fit image 110211.tif to the current display extent. Toggle on and off the tif in the Table of Contents or use the L key to toggle the visibility of the image. Zoom into the area of the basemap where the image needs to be georeferenced. Notice that there is a rotation on the scanned image.
Click on the Rotate tool; you will notice a yellow dot with a green sliced circle around it. The Rotate functionality is incredibly useful and easy to use to reposition the .tif image to the basemap reference data. With it enabled, click the image to toggle the transparency, and click and drag inside the green circle to rotate.
The Rotate tool is easy to use with it’s bright green dial-like circle icon.
Use the Move tool to reposition the .tif image to better align with the basemap. Use the Scale tool to resize the .tif, so that it’s in the correct dimension compared to the basemap. Use a combination of these tools to best align the 110211.tif image to the basemap.
The 110211.tif raster and Imagery basemap are roughly aligned using the Prepare tools.
Control Point Creation
When you’re confident in the placement of the .tif image, you can start creating the Control Points to link the 110211.tif raster to the basemap. Spread out the locations of your Control Points across the image for optimal results. The number of Control Points you need to create depends on which transformation you plan to use to transform the image to the map coordinates. The first-order polynomial transformation is commonly used to georeference an image, and anything higher than third order are rarely needed.
In this case, we’ll use the first-order, which means we’ll need a minimum of three Control Points. You can confirm the chosen transformation by clicking on the Transformation button in the Adjust portion of the toolbar. For more information on transformation methods, please see the transforming the raster documentation.
If you have the Auto Apply button enabled, the transformation will be automatically applied to the .tif image as the Control Points are added, removed or modified. Create five Control Points, and then open the Control Point Table and review the Residual column. If the Residual for a Control Point is particularly large, remove it and add a new one.
Do not press the Reset button to remove a Control Point because it will remove all of your Control Points and reset the image back to its original location. Use the Delete button in the Review Section to remove unwanted Control Points. Similarly, be careful using the Undo function. In some circumstances, it will remove the entire image from the map.
The Control Point Table provides information about the Control Points created to align the imagery.
In the final step, we want to permanently transform the 110211.tif image to the spatial reference information of the basemap. Recall that when you georeference your raster data, you define its location using map coordinates and assign the coordinate system of the data frame. Since we are georeferencing this image to an ArcGIS Online basemap, we’ll be applying the projected coordinate system WGS 1984 Web Mercator Auxiliary Sphere to 110211.tif. You have two options in the Georeferencing toolbar to persist the georeferencing:
- The Save button stores the transformation information to the existing 110211.tif file
- The Save as New button creates a new raster dataset with the georeferencing transformation applied
Whichever method you use, the resulting raster dataset will now preserve the spatial reference of the dataframe as well as the georeferenced transformation, allowing you to align your other spatial data for use in cartography and spatial analysis.
If you already have a referenced raster dataset, you can use the Auto Georeference tool. This tool will create the Control Points for you based on spectral signatures, so it best works on aerial and satellite data using rasters that are as similar as possible.
What do you think about the new georeferencing tools in ArcGIS Pro? Did you find this blog post helpful? Leave your comments below.