The goal behind our Basemap Georeferencing Workflow is to correlate pixels in the imagery with their corresponding real-world GPS coordinates. The map above is an example of aerial imagery being layed over a public basemap. Zoom in and see the difference in the available resolution. Use the control in the top right to hide the georefereced layer to see how things line up.

This alignment is acheived by the user creating correlations between a few images and an existing map. These correlations are called Ground Control Points, or GCPs. In creating these GCPs, a bit of precision is required to carefully associate objects in the images with a point on a basemap. The basemap will likely be of much lower quality than your images and is probably why this is being done in the first place, remember...

The important thing is to identify points in the images that are well represented in the basemap. A little bit of approximation is OK, but try to get as close as possible. Being off by a couple of pixels will likely still work out, but more than 5 pixels or so is not. Remember: garbage in, garbage out.

Image Controls:

  • Zoom In - Scroll or click
  • Zoom Out - Scroll or click while holding Shift key
  • Define Point - Right Click

Basemap Controls:

  • Zoom In - Scroll or + key
  • Zoom Out - Scroll or - key
  • Define Point - Right Click

A GCP is created by defining of a pair of markers: one on the image and one of the basemap. An Anchor Point consists of two GCPs that share an exact GPS location. This means that an object that is present in at least two images should be defined and assigned the EXACT same location coordinates. The "Reuse this location" button is a shortcut for this. Our process requires the definition of at least three Anchor Points in addition to at least one standard GCP.

Image Point Selection
Basemap Point Selection

GCP Creation Dialog

Sift through your images to find features that are well represented in your imagery as well as the basemaps.

Identify at least 3 objects that are the following:

  • Present in at least two images (This is where image overlap comes in.)
  • Present in reasonable detail on a basemap (features on the ground, NOT man-made structures or vertical features like trees)
  • Reasonably spread throughout the map being created (not right next to each other)
  • Not in a straight line.
  • Inside the flight path envelope. GCPs that are outside the interior or the area defined by the flight path risk being cropped out of the map.
Good Anchor Point
Good Anchor Point in another image

Anchor Points give Maps Made Easy the basis for aligning a created map with the world. The created map will only be as accurate as the Anchor Point entry. Sloppy Anchor Point entry can result in dramatically skewed results.

For each pre-selected object:

  • Click "Add New GCP".
  • Click Choose File and select one of the images in which the object is present.
  • Zoom in to accurately locate the object in the basemap. Multiple basemaps are available to try or you can use your own previously created map as a basemap.
  • Right click on the object in the basemap to designate that specific location. A marker will be drawn. Adjust as needed.
  • Zoom in and right click on that same object in the image. Make sure that the markers represent the exact same location.
  • If everything looks good, click "Save".
  • Click "Add New GCP" again to create the other GCP to be used in the anchor point.
  • The basemap location marker that was defined on the previous GCP is drawn again.
  • Click Choose File and select another image (not the same as before) in which the object is present.
  • Press the orange "Reuse this location" button to use the EXACT location again.
  • Zoom in and right click on that same object in the image. Again, make sure that the markers represent the exact same location.
  • If everything looks good, click "Save".
  • The table should be shaded green to show that these points represent on of the 3 required anchor points.
  • Repeat this process for at least two more anchor points.

Note: more Anchor Points or GCPs is not better. Anchor Points and GCPs do not affect the reconstruction and are only used to align the reconstructed model with the real world. 3 really good anchor points are better than 10 OK ones.

This is what a properly assigned GCP table should look like:

GCP Table with colors denoting Anchor Points

It sounds complicated but it really isn't. Check out our videos to see how quickly it can go. The workflow wizard will keep track of when enough GCPs and Anchor Points have been created and allow you to advance when all criteria is met. Good luck!