In last month’s edition of the Geospatial Frequently Asked Question (G-FAQ), we started our discussion on ground control points (GCPs) and their relationship to orthorectification. Specifically, our focus was on determine the number of GCPs required to create an accurate orthoimage and then selecting the locations for these points. In this edition of the G-FAQ, we conclude the discussion of GCPs by offering tips on collecting them in the field, recording metadata about each and then applying them to your orthoimage.
As a quick reminder, our focus in this two-part G-FAQ edition is on these questions:
What are ground control points and how do they improve the accuracy of high resolution satellite imagery? How do I decide on the number required and the locations of these ground control points? What best practices should I follow in the field when collecting ground control points?
In the October edition of the G-FAQ, we learned that ground control points are locations on the planet with a precisely known X, Y and Z position. When these control points are photo identifiable in high resolution imagery, they can be used to improve the data’s accuracy through orthorectification. Starting in August, we discussed the basics (and some of the nuances) of orthorectification, you can read more about the process here. In last month’s G-FAQ, I introduced an equation to help you determine the number of GCPs required for orthorectification and also looked at the ideal distribution of these points. This month, we pick up the discussion with a focus on collecting GCPs and then applying them to your raw high-resolution imagery.
Step 3 – Collecting Ground Control Points in the Field
Now that you have determined the number of GCPs required and their general locations, it is time to head out into the field to collect this data! Here is a check list to be sure you are ready for your trip out to the field and then some advice on what to do once you get there.
- First things first, it is time to pack your gear for the field. Here are some items you will want to be sure are included in your backpack:(1) A differential GPS unit or conventional surveying equipment. Whatever you chose, be sure it can achieve accuracies equal to half the imagery’s pixel size or better.
(2) A tripod for the GPS unit (or surveying equipment) with a precisely measured height to the bottom of the GPS while it is locked on the tripod. Be sure to record this height in a safe place as you will need the information later.
(3) A camera to take photos of the GPS-tripod in position, collecting control data.
(4) Back up batteries or alternative power sources for the GPS unit and camera.
(5) A laptop with the screengrabs of the approximate GCPs locations loaded on it as well as a copy of the raw imagery files.
(6) A premade document on the laptop to record the position information (i.e. X, Y and Z coordinates) reported by the GPS unit. We will discuss the format for this positional information in more depth below.
- Now let’s move to some tips for working in the field to collect GCPs. First and foremost, whatever you do, your safety is the number one concern! If you have selected GCPs that are on private grounds, be sure to request permission before you enter their lands. And if you have selected locations for GCPs that are unsafe when you get to the field, chose an alternate location that is safe (see some tips below on changing the location of GCP sites).
- When you have decided on the final location for each of your GCPs, set your tripod and GPS unit in place and let it get a lock on the number of satellites it needs to achieve the accuracy you desire. While you are waiting on the GPS unit to lock its position in, you will need to take at least two photos of each GCP. And be sure to take photos of the GPS unit-tripod from different angles. These photos will be crucial when you get back into your office and try to relate each GCP to the exact pixel it represents in your imagery.
- If you do change GCP locations in the field, it is a good idea to mark an imagery screengrab with the new position. If the new locations are not on the original set of screengrabs you made, create a new screengrab(s) and mark the new GCP site(s).
- In order to keep your GCPs organized, it is a good idea to create a spreadsheet to record the metadata on each control point you survey. Here are the items we suggest you add to the spreadsheet:(1) The model and number of the tripod, GPS unit and/or surveying equipment used.
(2) A point identification number.
(3) The X, Y and Z coordinates of the point.
(4) The projection (i.e. datum, coordinate system and units) used to survey the GCP.
(5) The estimated error or XYZ covariance matrix for each point.
(6) Written description of the general location of each point.
(7) Additional notes including details on how photos and imagery screengrabs of each point are named.
If you would like to use a spreadsheet I created, you can download it here.
Step 4 – Applying Your Ground Control Points to Imagery
Now that you have collected your set of GCPs, it is time to get back into the office and apply them in the orthorectification process. In short, what you need to do is locate the exact pixel that each of your GCPs relates to – that is why the photos you took in the field are so important. As you find this exact pixel and apply GCPs, be sure look at the RMS error (sometimes called residuals) of each point you add as one bad point could degrade the accuracy of the final product. Keep in mind that residuals are computed on a global best fit basis so a point with a high residual might not actually be bad. It might just be isolated as isolated GCPs often have higher residuals than will clustered GCPs. If possible, having a final residual value smaller than your pixel size is the goal. And one final word to the wise with regards to GCPs, if you can, be sure to leave out several points to test the accuracy of the final orthoimage you have created.
Well, that brings us to the end of our discussion of orthorectification and ground control points. Hopefully you learned a bit along the way in this four part discussion!
Do you have an idea for a future G-FAQ? If so, let me know by email at firstname.lastname@example.org.
Find Out More About This Topic Here
- Michigan Technological University – Photogrammetric Control Survey
- Penn State University – Land Surveying and GPS
- Texas A&M University – Remote Sensing in Geosciences
- University of Arkansas – Control Point Documentation
- Western Oregon University – Image Registration
Brock Adam McCarty