Section 1 – Raster Editing
Many of the raster files that we deal with in ArcGIS have a geographic projection assigned to them and they will position themselves in the correct location when added to a map. This is not always the case however. When faced with an image that needs to be moved or corrected ArcMap offers a number of tools that can be used to make these adjustments.
In order to be successful in using the raster editing tools you will need to put some thought into what your desired outcome should be!
You are going to want to add some layers of data to your map that will help you in this process. Suggestions would be things like road networks for the area where you will place your raster, rivers and/or lakes, railroads, etc. These are items that will effectively help you establish reference points that will be used to move and alter your image to fit your needs.
Image Tools
The Image Tools that are located in the Design Tools custom toolbox can be used to perform a number of editing processes.
Flip
The Flip tool is useful if you have a raster image that needs to be flipped from top to bottom. It will flip the image using the horizontal center axis.
Mirror
The Mirror tool will flip the raster image from left to right using the vertical center axis.
Mosaic
The Mosaic tool allows the user to convert multiple raster images into a single raster dataset.
When using this tool the Target Raster needs to be an existing raster dataset. This can either be an image containing data or an empty raster dataset. Empty raster datasets can be created within a File Geodatabase using ArcCatalog.
Project Raster
The Project Raster tool is used to re-project a raster image from one projection to another.
Rescale
The Rescale tool resizes a raster image by X-Y factors input by the user.
Rotate
The Rotate tool pivots a raster around a specified point by a user defined angle.
Shift
The Shift tool moves a raster image to a new geographic location based on user defined X-Y values.
Warp
The Warp tool adjusts a raster image based on user defined control points.
Georeferencing Toolbar
The Georeferencing toolbar contains a series of tools that allow you to move and alter a raster image to fit your map geometry. These tools will also assign a projection to the newly edited raster so that it can be used later in any map with correct location.
The Georeferencing toolbar can be launched by choosing it from the View>Toolbars menu option at the top of the screen.
The Georeferencing toolbar will automatically recognize any raster images that are currently in your map’s table of contents and those images will be listed in the Layer drop-down list.
1. Pick the desired raster image from the Layer drop-down list.
3. From the drop-down menu on the Georeferencing toolbar choose the Fit To Display option.
This process will move and resize your image according to the area displayed on your screen.
You may or may not see your image show up depending on how your layers are arranged in your table of contents. If your image is not displayed, simply click and drag your image above any layers that are covering it.
At this point your image will not be a perfect fit to your map geometry.
If you do not like how your image is sized to your map, you can choose the Reset Transformation option in the drop-down menu.
4. Now select the Add Control Points tool.
You will use this tool to warp your image to fit the geometry in your map. To use this tool correctly you will want to pick points on your image that directly correspond to points in your map. Things like centerpoints of intersections, the end of a bridge, even a point along the curve of a road.
Points toward the corners of the image will help warp the image faster.
5. Left-click a point on your image to establish a control point.
6. Left-click the corresponding point on your map geometry.
Your image should now move and resize to fit this first control point.
7. Move to the opposite corner of your image and repeat this process for another point.
Your image should continue to adjust to each control point you establish. You can set as many control points as necessary to make your image fit your map, however 2 control points are the minimum.
If you do not like how your image is adjusting to your control points, you can choose the Delete Control Points option in the drop-down menu to start over.
8. Once your image is correctly sized and adjusted, choose the Update Georeferencing option from the drop-down menu on the toolbar.
Choosing this option creates a couple new files that are saved in the same directory as the raster image. These files give your raster image position and size information.