Home-Brewed Ground Control Markers

By | May 25, 2020

DIY UAV Ground Control Markers

This article has an update 20200525.
Behold the GeoVelo Mark 1 – Mod 0 Ground Control Marker!
Click the image to enlarge.

Hey, Kids! It’s All Drones These Days!

Thanks to their availability, quality, and increasing ease of use, many GIS organizations are collecting aerial imagery with unmanned aerial vehicles (UAVs). With the abundance of high-resolution imagery coming from UAVs, members of the GIS tribe often find ourselves using ground control markers (GCMs) to georeference the imagery we are collecting.

Judd Slivka and members of the University of Missouri Drone Journalism Team preparing to admire my new ground control markers from the sky. They also collected some imagery while they were up there.

What Are Ground Control Markers Used For?

In a nutshell, GCMs enhance the accuracy of a UAV’s imagery. Of course, some conditions must be met to ensure accuracy: First, a set of GCMs must be placed within or near the UAV flight study area, and placed in a manner where they will be visible to the UAV’s sensors. Second, their location must be accurately determined by a global navigation satellite system (GNSS) before the flight. When the imagery from the UAV flights are then processed, knowing the GCMs’ locations will greatly increase the accuracy of the imagery after georeferencing.

ArcMap screenshot of an actual GCM deployment for a recent job.

Why Make Your Own GCMs?

Why anyone would make their own GCMs is a perfectly reasonable question. I was motivated to make my set because I had a pending job where my client engaged another organization to make a series of UAV flights. The UAV operators didn’t have their own GCMs, and there were not many physical features in the study area that lent themselves to georeferencing. So, we needed some GCMs.

I looked at commercial units, and none of their designs appealed to me. The commercial unit in the image below looks like it would work well, and at $6.77 a unit, it is reasonably priced. But the available commercial versions lacked a few features I thought might be nice to have. So, I designed and built my own. In this article, I share my process in case you find it useful for your own projects.

Research GCMs Before Committing to a Design

My first step in this project was to look at the GCMs others were using and their reasoning behind what makes for good GCM design.

  • GroundControlPoints.com has a great discussion of the topic with lots of good graphics, so consider giving it a look.
  • Precision Mapper focuses on how to deploy your GCMs.
  • Let’s not forget that everything under the sun is discussed on Reddit.

Create Your Custom GCMs

  • Design Features: I think triangles show up better than square features in imagery. Triangles also require one less grommet, nail, and washer, so I made my GCMs triangular.
  • Branding: I wanted my own company information on the GMCs.
  • Numbering: I wanted each GCM to have a unique number so that, when I entered the location information into Esri’s Collector, it would allow for better geospatial chain-of-custody control claims in court.
  • File Format: I have several different graphics programs, but my plotter plays best with Adobe files, and I drew the GCMs in ArcMap. Then I exported to a PDF.
  • Sharing is Caring: Download my design in the next section …


Sharing is Caring, So Download My Design: Here is the zipped mxd file and the PDF.

Tools List

  • Hammer: All great projects start with a hammer. I used a carpenter’s hammer to punch holes and form the grommets.
  • Software: You have to design your GCMs using something that you can send to your plotter.
  • Plotter: Use a large format plotter (printer) that accepts a vinyl banner material.
  • Grommet Kit: This kit consists of two dies, a cutter, and an inserting punch. You can also use a fancy press. But for this project, a $5.00 grommet kit is more than adequate.
  • Expendable Surface: When you are cutting holes in your media and punching your grommets down, you don’t want to do it on anything you want to keep nice. I used a bit of scrap laminate flooring as my pounding surface.
  • Scissors, Razor Knife, and Paper Cutter: All came in handy when cutting the individual GCMs from the roll.

Parts List

  • Media: HP Opaque Scrim or a similar vinyl banner material on a roll. I used the house product sold by Tierney Brothers. I’ve had a good experience working with them for plotter supplies.
  • Grommets: I used 0.5-inch brass grommets from Menards. They are also available at Walmart and just about everywhere that sells hardware.
  • Spray Sealant: To protect the ink and add some additional waterproofing, I used Krylon’s Crystal Clear Acrylic Coating 1303A. The stuff works great but is also on the expensive side. Several photos below you can see my experiments using several different coatings. The Krylon is easily the best sealant and protector.
  • Utility Flags: When you are recovering your GCMs after the flight, it helps to be able to spot them from a distance. I found white wire utility flags allowed me to locate the GCMs from a further distance than just looking for them on the ground.
  • Nails and Washers: You don’t need nails and washers to create the GCMs. You need them to hold the GCMs down when you are in the field. I used 1.5-inch fender washers marked “6GC” and some beefy 4.5-inch-long nails. That combination worked great when used on natural surfaces. To my surprise, I bent the nails when I tried to hammer them through asphalt. Use the sandbag tip mentioned below to affix the GCMs to Adamantium-type surfaces.
Here is how the nails and washers are used with the GCMs.

Production Sequence

  1. Design your GCMs.
  2. Load your media in your plotter.
  3. Send your file to your plotter.
  4. Spray-seal the GCMs, then allow them to dry.
  5. Cut your individual GCMs from the printed roll.
  6. Install your grommets.
  7. Deploy to the field and impress everyone with your craftsmanship.
My trusty HP Z-5200 printing the first batch of GCMs.
I should have sealed the GCMs before I cut them out of the sheet.
I tested every clear-coat product I had in my vast chemical cabinet. Go with the Krylon.


  • Each GCM weighs 1.25 ounces without nails and washers.
  • Three washers and three nails weigh a total of 3.5 ounces.
  • Thus, the total weight you have to carry with you into the field to deploy a single GCM is 4.75 ounces.
  • Because of the way georeferencing works, it doesn’t make sense to deploy more than six GCMs per study area.

Deploy Your New GCMs in the Field

The process of putting down GCMs:

  • Flight Test: Perform a test run with the UAV flying at the planned mission’s height and with the sensor package set to the resolution you plan to use for the job. Don’t forget to account for weather and time of day. Then pull the imagery into your GIS and make sure you can see and center your cursor on your GCMs.
  • Plan: Plan the job ahead of time and ensure your GCMs will be in view of the UAV’s sensor package. GCMs in dense forest may not be visible, for example. Start with a map of the study area and then determine where the GCMs are best deployed
  • Deploy: Place your GCMs before the UAV flight. As you are placing the GCMs, use your GNSS unit to collect their location.
  • Recover: After the UAV flights are complete recover, your GCMs.
You place your GNSS on a stick in the center grommet hole. Then take your readings. Someone on our team drove over this GCM at least four times. It stayed put and didn’t rip.
A member of the UAV team I worked with on this project named my Mapping Grade Stick of GeoLocational Precision (MGSGP) “Excalibur.” I like it.

When You Get Back to Your GIS Workstation

This is the easy part. Download your GNSS’s field data and start your normal georectification process.

This is a screenshot from ArcMap showing three GCMs as they appeared in the imagery collected during the test flight. Notice the black has disappeared from the image. I’d love to know why.
A screenshot of a GCM from 150 feet.

Lessons Learned

  • Starting with Pretty GCMs: A freshly printed GCM’s ink will smudge and smear if you don’t coat the GCM with some sort of sealant. So, seal before you cut a GCM from the sheet.
  • Reflective Tape: When I first designed my GCMs, I thought I might be recovering them after dark if the UAV work went to dusk. I thought that adding some reflective tape would aid in their recovery. This extra precaution remains unproven.
  • Flexible Media: The vinyl used to make the GCMs is very flexible and can be stuffed in a backpack or cargo pocket with no ill effect.
  • Washable: I haven’t determined if the GCMs are dishwasher safe, but I can tell you that they clean up easily with a little soap and warm water.
  • Observability: Higher flights and lower sensor-package resolutions require larger GCMs. If you can’t see the GCMs in the UAV imagery, then you wasted a day. This is why you should always perform a test flight until you are confident you know the GCMs will be visible in the imagery.
  • Durability: The vinyl roll material is tough, very rip resistant, and holds its color. To see how tough this stuff is, I’ve had a GCM out in my backyard for about six months. I’ll occasionally post photos of it as it ages.
  • Hard Surfaces: MikeM asked how I would attach these GCMs to a rock surface or place them over a U.S. Geological Survey (USGS) benchmark. For hard surfaces, make three small sandbags per GCM. Then use twine to attach the GCMs to the sandbag anchors. They won’t go anywhere. To mark a USGS benchmark, first take your GNSS reading at the benchmark. Then put the GCM over the benchmark, making sure your center grommet is centered on the dimple in the center of the benchmark.
  • North Arrow: MikeM also asked if the north arrow was of any practical use. My thinking was that, if the GCMs were always aligned with their tip pointed toward north, it would be easier for the GIS operator to accurately place his or her crosshairs dead center on the GCM’s image during the georectification process.
  • The Illegal Practice of Surveying: TS asked if “[Are] these ground locations being collected by certified land surveyors? The world of drone data collection has definitely put a twist on the traditional realm of aerial imagery. If it was aerial imagery, you wouldn’t dare not have a surveyor do that work.” Because the work we were doing was mapping, and it didn’t involve the OVERLY BROAD AND VERY LIKELY UNCONSTITUTIONAL enumerated list of items that only surveyors may perform in the state of Missouri, and because the results of our work will never “affect real property rights” or be represented to others as a land survey, I’m confident that we were well outside of the scope of the unlawful practice of surveying per RSMo §327.272.

Points Left to Ponder

Sensor Resolution and Observability: In the future, I’ll come back and insert some discussion of GCM size and UAV sensor resolution settings. Or I’ll insert some links to work by others on the topic.

Durability: I’ll also post some photos of my aging backyard GCM.


I wouldn’t have needed to create a set of GCMs if it wasn’t for several of the projects I was working on for the Missouri Humanities Council. They are a great client, amazing nice people, and I consider myself fortunate to work with their diverse team of talented professionals. If you love Missouri history and the humanities you should subscribe to their quarterly magazine. Its free!

I also want to thank Judd Slivka, a drone pilot certified by the Federal Aviation Administration (FAA) and a former Professor of Convergent Journalism at the University of Missouri’s Reynolds School of Journalism. Judd worked with me to help test the GCMs, and he encouraged me to share this information with the geospatial community. He can be reached at his new venture at Judd@SeeArizoneByAir.com. His company is seearizonabyair.com

Link of the Week

Pretend you are testifying in court and the attorney who wants to discredit your work starts asking about the accuracy of your maps and their underlying data sets. You’ll be glad you familiarized yourself with this short two-page discussion of the topic: the USGS’s Map Accuracy Standards. Of course, this PDF is just the tip of the map-accuracy iceberg. I’ll post more about map accuracy in the near future.

If you enjoy this blog and know someone else who might enjoy it as well, please share the link.

Topics I’m considering pontificating about:

  • Legal cases that ended up dragging geospatial workers into court
  • Human spatial blindness
  • Maps that need to be created
  • RSMo § 67-1850 Missouri’s Public Records $ales $tatute

About the Author

You can view Chris Dunn’s third person singular CV here. Basically, he’s a geographer with a law degree, a bar card, and a kick-ass GIS workstation. Email me at Chris.Dunn@GeoVelo.com


One thought on “Home-Brewed Ground Control Markers

  1. Chris Dunn Post author

    Comment by Jon Cole, Merebrook LLC Land Surveying – Posted by Chris Dunn


    I finally got around to reading your newsletter article. It looks very good. Nice job.

    I do have some comments for you regarding GCMs on hard surfaces. I recommend painting them on paved roadways. You can use whatever kind of paint you want , even leftover latex paint from a house project works. You just need to put it down in a recognizable pattern. Free-handing it with a roller or brush and a tape measure works quite well, or you can get fancy with a template and spray paint. Most of the paint will be worn off by weather and passing vehicles in a few days or weeks, so plan accordingly.

    Also, surveyors commonly use 60d nails as traverse and temporary points. The are inexpensive and beefy enough to hold up to most pounding. If you need something more substantial for going into asphalt, concrete nails are the cheap version, or you can get the much more expensive MAG nails or PK nails, which are easier to find with a magnetic locator if you need a semi-permanent spot marked. Both types of nails come in several lengths, giving you even more options.

    Jon Cole, PLS

    Merebrook LLC Land Surveying
    914 N. College Ave., Ste. 1
    Columbia, MO 65202
    (573) 777-3564

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