DroneDeploy

Ground Control Points (GCPs): Capturing & Processing

How to Successfully Use Ground Control Points for Drone Mapping

How to Successfully Use Ground Control Points for Drone Mapping

Ground Control Processing Access

GCPs are available to Enterprise Level Customers and are also available to Business users for $75 per map. If you are currently on a Pro or Business plan and are interested in GCPs, please contact support@dronedeploy.com. We can help determine if GCPs are the best option for your mapping project.

Ground Control Points are an effective way to increase the absolute accuracy of a map (more about mapping accuracy here). To ensure that your GCPs process accurately and efficiently, it's imperative that DroneDeploy receives specific pieces of information for your project. Please review this guide prior to adding GCPs to a map.

GCPs that follow this guide should take 1-3 business days to process after DroneDeploy has received the information outlined in Step 4.

However, if your GCP request meets the following conditions, processing time will be greatly reduced!

  • Submitted between 5am-11am PST, Monday-Thursday (GCPs are not processed outside business hours, on weekends, or Holidays)
  • GCP locations are measured in WGS84 (EPSG Code: 4326)
  • Requests contain 10 GCPs or less
  • Requests contain 1000 images or less
  • GCPs are unobstructed and clearly marked with an X or checkerboard larger than 4 sqft.

Need a Refresher on Projections, Reference Systems or Sea Level?

Check out this guide on the many intricacies related to global accuracy to help better understand and improve upon your map's accuracy.

Please follow the instructions outlined in this guide, when this process is not followed, it often creates large delays in getting you a correctly processed GCP map.

1. Create at Least 4 Quality GCPs on the Ground

The first step in processing GCPs is to create clear GCP markers evenly distributed around the area you are mapping.

An ideal GCP marker can be as simple as two intersecting lines. The goal is to create a visible feature on the area that is being flown. The GCP should be visible on the map at the given altitude that the mission was performed. Remember, if you cannot clearly see the center point of the GCP in your map photos, it cannot be accurately marked during processing. If your GCP is obscured, poorly marked, or if you cannot confidently place your cursor on your photos within an inch of the point where the GCP was measured with GPS, it does not matter that your GPS took a centimeter or millimeter accurate reading, your GCP will only be as accurate as the center can be marked.

Adding a center mark to the GCP, as seen in the images below, can also eliminate confusion as to where the center point is located.

Example of a well constructed GCP. The marker is large enough that it is visible from far away, but the location will be measured from the center to increase accuracy.

Example of a well constructed GCP. The marker is large enough that it is visible from far away, but the location will be measured from the center to increase accuracy.

If you cannot mark your GCPs with paint there are a variety of low-cost ways to make excellent markers with items from the internet or home depot. These sheets of roofing rubber ($4.66 each) with these vinyl stickers ($1 per 6" x 12" sheet) can be used to make a simple checker pattern but they need to be staked into the ground if its windy.

These rubber and vinyl markers end up costing $5 each and are extremely weather resistant.

These rubber and vinyl markers end up costing $5 each and are extremely weather resistant.

A fantastic DIY approach we have seen is a 12" bucket lid stuck on a white plastic 14x14 access panel from Home Depot

A fantastic DIY approach we have seen is a 12" bucket lid stuck on a white plastic 14x14 access panel from Home Depot

A less weatherproof but disposable idea is marking cardboard cake circles (shown below) with a sharpie and securing them with a 6" magnetic nail for easy recovery.

Placement of GCP points should be scattered throughout the area of interest being mapped. If the area being mapped has noticeable elevation changes, make sure your GCP placement (hills, mines, valleys etc.) accounts for this by placing them in the different elevations on a relatively flat surface. Ensure that the region of interest (where you want the map accuracy to be highest) has GCP points surrounding it, as well as scattered throughout the region. When placing GCPs around the perimeter DroneDeploy recommends a 50-foot buffer zone between the edge of the map and the GCPs location. This will ensure that there is enough image coverage to carry out the reprocessing.

Measuring GPS from the center of the point helps increase accuracy.

Measuring GPS from the center of the point helps increase accuracy.

Tips:

-Take the coordinate from the center of the intersecting lines' GPS coordinates.
-Use an RTK GPS system for the most accurate reading.
-Fly with a high-resolution (20MP or greater), mechanical shutter camera like the DJI Phantom 4 Pro.

How many GCPs and where to place them?

DroneDeploy requires a minimum of 4 GCPs. No more than 10 are usually needed for larger maps. Please note that if you decide to use more than 10 GCPs, processing time for your map will increase. We recommend 5 GCPs located in the 4 corners of your map and the center as shown below. Again, make sure your GCPs are at least 50 ft from the boundary of your map.

In this example, the subjects of interest are the 2 star shaped buildings so we place our GCP as best we can in the corners and center.

In this example, the subjects of interest are the 2 star shaped buildings so we place our GCP as best we can in the corners and center.

In the example the subject of interest is not a square so we distribute the points as best we can in the corners and center. You can also see some other measurement annotations.

In the example the subject of interest is not a square so we distribute the points as best we can in the corners and center. You can also see some other measurement annotations.

2. Conduct Your Flight

GCP flights are flown just like any other flight. The only difference is that you are now capturing GCP targets in your imagery that will later be used to increase the accuracy of your map. Make sure that your camera is in focus and setup correctly. If the GCP targets are not in focus the process will not result in the same levels of accuracy. It is always a good idea to make sure the area you will be flying that day has good weather. High-winds, low-clouds or precipitation can make flying and mapping difficult. For more information, please see this article: Making Successful Maps.

We highly recommend flying with a high frontlap and sidelap (80/80) and below 200ft if you a looking maximize the accuracy of your GCP map. We also recommend flying with a high-resolution (20MP or greater), mechanical shutter camera like the DJI Phantom 4 Pro. With a Phantom 4 Pro flying below 150ft you can capture images with a ground sampling distance of ~0.5 inches per pixel. By also including RTK GCPs you can achieve the best ASPRS RMSE's ranging from 2cm-5cm. This is then ASPRS certifiable in horizontal classes of 2.5cm-7cm. See the full ASPRS accuracy standards here.

3. Upload Your Images to DroneDeploy

Follow typical protocols in Uploading Images to DroneDeploy.

4. Add GCPs to Your Map

Once you have selected your images (but before you start processing your map) select the GCP dropdown.

Download the GCP file template and add your coordinates to the template. The template will contain example coordinates for you as a guide.

Before we continue, here is what you need to know about EPSG codes

An EPSG code is a standardized definition of the spatial reference system used by your map. The code tells us 3 things:
1) The Coordinate System (ie. where to place the origin, or geodetic datum)
2) The Measurement Units (ie. Feet or Meters, Northing/Easting or Lat/Long)
3) The Map Projection (The earth is a sphere but maps are flat! We use map projects to convert between the two)

Find a full explanation of the information above at https://www.groundcontrolpoints.com/

WGS84
DroneDeploy strongly recommends using WGS84 for GCP projects as processing time will be greatly reduced. WGS84 (EPSG: 4326) is the standard that our GPS and GNSS networks operate on. WGS84 uses Lat/Long, measures elevation in meters, and uses an ellipsoid coordinate system with no projection. More information on WGS84 and mean sea level here.

State Plane Coordinate System
Other common EPSG codes are from the State Plane Coordinate System. SPCS is a set of 124 geographic zones or coordinate systems designed for specific regions of the United States. Each state contains one or more state plane zones, the boundaries of which usually follow county lines. These are all based on the North American Datum of 1983 (called NAD83). NAD83 uses Northing/Easting and measures elevation in US survey feet but depending on the specific state plane it can use many difference coordinate systems and map projections.

Make sure you ask your GCP surveyor what EPSG they are using and request WGS84 (EPSG 4326) before GCPs are measured

If you are a drone service provider it's also important that you ask your client if they require mapping data in a specific EPSG code. You must have a valid EPSG code to use GCPs with DroneDeploy.

We cannot process maps that do not have an EPSG code
Unfortunately, if your GCPs are measured in a local or arbitrary coordinate system that is not listed in the EPSG Geodetic Parameter Registry, we cannot process them.

Now, let's get back to uploading GCPs

Enter your recorded GCP coordinates into the template. Make sure your columns are Northing/Easting or Latitude/Longitude and please check that the values are not flipped the other way around when entering them into the template or we will not be able to process them. Also, make sure that your elevation units reflect the same units as your selected EPSG code. If you are using WGS84 your elevation should be in meters. If you are using a State Plane coordinate system your elevation will likely be in US survey feet.

This template document contains example coordinates. You can include your coordinates in State Plane or WGS84 Latitude / Longitude. Please make sure that the units of your coordinates match the units of the EPSG code.

This template document contains example coordinates. You can include your coordinates in State Plane or WGS84 Latitude / Longitude. Please make sure that the units of your coordinates match the units of the EPSG code.

Once you've added your GCP information to the template, save your .cvs it and then upload it into DroneDeploy.

Upload your completed GCP Excel File into DD as a .CSV file

Upload your completed GCP Excel File into DD as a .CSV file

Next, add your EPSG code. A full list of Codes can be found at Spatial Reference. Again, most GCP maps are in WGS84 or one of the NAD83 (typically not HARN, CSRS, or NAD27) State Plane Coordinate Systems.

Add your EPSG Code

Add your EPSG Code

Once you have finished the above steps, click "Confirm GCPs". You are now ready to start processing your images by clicking the blue cloud in the bottom right corner of your screen.

5. DroneDeploy Processes Your Map with GCPs

DroneDeploy will begin processing your GCP map. We may contact you if their are any issues or questions with the information provided. Please allow 1-3 business days for processing once we verify the data is correct. If more than 10 GCPs are submitted, the processing time will increase.

6. Your Map Fully Processes with Your GCPs

Once your map has been completed, we will send you an email with your map and the accuracy information for each GCP. The map will be available on your account dashboard on DroneDeploy.com.

Ground Control Points (GCPs): Capturing & Processing

How to Successfully Use Ground Control Points for Drone Mapping