In our last two editions of the Geospatial Frequently Asked Question (G-FAQ), we focused on the basics of positional accuracy with regards to high resolution satellite imagery, and specifically on orthorectification. In these G-FAQs, we learned that in order to orthorectify an imagery to the highest accuracy possible, an elevation model, RPCs or a camera model and ground control points (GCPs) are required. In this two part G-FAQ series then, we expand upon the topic of ground control points with a discussion of best practices for selecting their locations and surveying them in the field.
Specifically, our focus in this two-part G-FAQ edition will be 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?
Before we being this G-FAQ, let me first frame the discussion a bit. As in the G-FAQ editions on orthorectification, our focus here will be on high resolution satellite imagery as in order to utilize GCPs, they must be clearly visible in the imagery source. In August and September, we learned that orthorectification is the process of taking loosely referenced imagery and tying it to its ‘correct location’ on the planet. In order to complete this process, an elevation model (i.e. a DEM) and RPCs or a camera model are required. However, to have a stated accuracy, GCPs are required whereby it is possible to measure the distance of a known feature in the orthorectified imagery, such as the corner of a sidewalk, from its true location on the planet. With this in mind, let’s jump head long into this discussion of GCPs, their importance, selecting appropriate locations and collecting them (safely) in the field.
Basics of Ground Control Points (GCPs)
Ground control points are locations on the surface of our planet with a known X/Y (e.g. latitude and longitude) and Z (e.g. height above mean sea level in meters). In order to use GCPs in the orthorectification of high resolution satellite imagery, they must be photo identifiable – meaning that the points must be clearly visible in the images you are working with. Here in the United States, there is a series of control monuments (also called benchmarks) established across the country, usually demarcated by small bronze or silver plaques. And while these monuments are extremely accurate, they cannot be seen in high resolution satellite imagery so they are not appropriate for use in orthorectification.
In order to use GCPs in the orthorectification process, they need to be extremely accurate. You cannot collect them with a low-priced handheld global positioning system (GPS) unit or with your smart phone. What is required is a survey-grade GPS unit which is often referred to as a differential GPS unit. These units work by locking on to multiple GPS satellites in space that then triangulate your position on the ground. The longer you wait, the more accurate your position will be calculated as more satellites lock onto the unit. The position of each GPS satellite is then verified and corrected by a series of ground stations that monitor their orbit, velocity and direction of movement.
Returning to the visual analogy used to describe orthorectification in our last G-FAQ series, if you consider imagery a balloon and a DEM as an accurate representation of the shape of the planet’s surface, then GCPs are the tie points used to attach the balloon to its correct location over the DEM. By using a network of evenly distributed GCPs that tie your balloon to the DEM, you can stretch the balloon over the shape of our planet and move it to its correct location with regards to its XY position. It follows then that the accuracy achieved by orthorectification is limited by the positional accuracy of the GCPs and the ‘trueness’ of the representation of the Earth’s topography (i.e. the DEM).
Step 1 – Determining the Number of Ground Control Points
The first step in obtaining GCPs for use in orthorectification is determining the number of control required for your area of interest. In general, we tell clients to collect at least 10 GCPs as that way you will have extra control to toss out if it is of low quality and/or use for accuracy testing; and if you can collect more than 10 points, by all means please do! More GCPs are required: (1) as your area of interest grows in size; (2) in areas with lots of terrain change; and (3) if you have multiple overlapping images to orthorectify. Here is an equation you can use to help determine the number of GCPs to collect:
Number of Control = 10 + (Area Covered in Square Kilometers / 25) + (2 x Number of Overlapping Scene Edges)
So if you had a polygon that is 150 sq km with two scenes that overlaps at a single edge, then you would want to collected at least 18 GCPs. Please keep in mind this equation is just a rule of thumb and is definitely not accurate in all cases, for example if the area is very mountainous.
Step 2 – Selecting the Locations of Ground Control Points
Now that you have determined the number of GCPs required for your dataset(s), it is time to pick out good locations to survey in the field. Here are some general tips to follow when selecting the locations of GCPs:
- Spread out your points as much as possible across the imagery as clustering GCPs results in lower accuracy as you move away from the cluster.
- Select at least two points per edge (if possible) where various images overlap.
- Be sure to select points that are in high, medium and low elevation areas by using a free DEM such as ASTER GDEM, USGS NED or SRTM as a guide.
- Choose features that are stationary and unlikely to move. Also choose flat features when possible as they have no lien in the imagery so their locations can be determined more accurately.
- Features with a high contrast between them and surrounding grounds will be the easiest to locate accurately in your imagery.
- Select features for GCPs locations that are big enough to easily see in your imagery but also small enough so that you can pinpoint the exact location down to a single pixel.
With a general sense of the locations to look for, the next step is to scan your high resolution imagery, pick out the features you will survey in the field and then take both a screengrab of the locations as well as a general latitude and longitude. The screengrabs and coordinates will help to orient you when in the field. Now, here are two lists of suggested locations for GCPs in both urban and rural areas:
Urban GCP Suggestions
- Corners and intersections of sidewalks
- Corners of pavement and parking lots
- Parking space lines and particularly the intersection of these lines
- Painted white and yellow center lines of roads (though these can have safety issues when surveying them in the field so use sparingly)
- Choice points at local high spots that might not have been apparent in the free DEM analyzed before you went into the field
- Avoid building corners if possible as their lien in imagery can obscure where the building exactly hits the ground
Rural GCP Suggestions
As you can imagine, finding good control in rural areas can be challenging, especially if it is very remote. Given this, orthorectified images in rural locations tend to be less accurate than those from urban areas.
- Edge of bridges where they meet a road surface
- Sides of roads and paths where they curve and/or intersect with other roads and paths
- Bottoms of telephone poles and/or electrical towers
- Fences when they can be seen in the imagery (it is often the shadow you see as opposed to the fence posts and rails)
- Livestock pens, though these tend to change locations more so than other rural features
- Road and stream intersections
- Stream curves or intersections with other streams
- Distinctly colored or shaped rocks
- Small shrubs
- Distinct clearings in trees
When all else fails, if your location is so rural that there is no possibility of finding appropriate locations for GCPs, then you can establish control in the field with painted wooden/plastic panels. The main issue with this technique is that you need to put these panels in the field before the imagery is collected so it is not an option for many projects. If you do want to put your own panels in the field for ground control, I suggest using a white or beige surface with a black X painted on it. The panels should be at least five times the pixel size so they will be obvious in your high resolution imagery. Place the panels in flat, open areas with no trees and away from roads so there is less risk of vandalism. You will want to obtain permission to put these panels on private lands.
In our next G-FAQ edition, we will continue this conversation on GCPs with a focus on surveying the points in the field and using them to orthorectify your imagery.
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
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