What are some best practices for documenting and sharing georeferenced images?

1 Choose a suitable image

The first step in georeferencing is to choose an image that is relevant, clear, and has identifiable features that can be matched with a reference layer, such as a base map or another georeferenced image. The image should also have a known scale, projection, and orientation, or at least some information that can help you estimate them. If possible, avoid images that are distorted, rotated, or cropped.

2 Load the image and the reference layer

The next step is to load the image and the reference layer into your GIS software. Depending on the software you use, you may need to create a new project, add a data frame, or set the coordinate system. The reference layer should have the same or a compatible coordinate system as the one you want to assign to the image. You can also adjust the transparency, color, and contrast of the image and the reference layer to make them easier to compare.

3 Add control points

The third step is to add control points, which are pairs of points that link a location on the image with a corresponding location on the reference layer. You can use landmarks, intersections, boundaries, or other features that are visible and consistent on both layers. To add a control point, you need to click or select a point on the image, and then click or select the same point on the reference layer. You can also enter the coordinates manually if you know them. You should add at least four control points, and preferably more, to cover the whole extent of the image.

4 Apply a transformation

The fourth step is to apply a transformation, which is a mathematical function that adjusts the position, scale, and shape of the image based on the control points. There are different types of transformations, such as affine, polynomial, or projective, that vary in their complexity and accuracy. You should choose a transformation that fits your image and your purpose, and that minimizes the error or residual between the control points and the reference layer. You can check the error or residual by looking at the table or report that the software generates.

5 Save and export the georeferenced image

The final step is to save and export the georeferenced image. You can save the image as a new file, or overwrite the original file, depending on your preference and the software options. You can also choose the format, resolution, compression, and metadata of the georeferenced image. Some common formats are TIFF, JPEG, PNG, or GeoTIFF. You should also export the control points and the transformation parameters as separate files, in case you need to modify or redo the georeferencing later.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?