![]() Most projects, especially where accuracy is tied to safety, the cheapest option is simply not acceptable. To solve the height offset we recommend adding one GCP to lock the digital twin into position. ![]() And also as you would expect, the cheapest option using the Phantom 4 Pro with no GCP’s clearly gave the worst results, with the digital twin actually off by multiple meters!Īdding RTK pushed results into centimeter accuracy, but was still burdened with a 1-meter height offset. As expected, the RTK technology wins hands down over the standard Phantom 4 Pro for accuracy. The results were surprisingly interesting. What it does is connect the GPS system to a base station on the ground that constantly sends corrections of a GPS error in real-time, usually within a centimeter. We can do this by using a relatively new technology to drones called RTK, real-time kinematics. Another option is to insure that the GPS data inside the photos is better. That “locks” where the images are located. One option is to measure some points on the ground using a very accurate GPS receiver and then marking those control points in the images. You can do this by using the GPS data inside those photos, but this is not always completely accurate. While the drone flies over your site and captures a multitude of photos, you need a way to position these photos in space. As mentioned before, the Ground Control Points are global reference points that are measured in advance of the drone flight. We actually did a lot more setups than these but are highlighting only the most noteworthy and most commonly used in the field. This will show how a premium GPS can function even without using any ground control points. Now we also tested the next generation drones that DJI released in 2018, which is the Phantom 4 RTK. The second option is the combination of a GPS drone, but adding five GCPs. This is a slightly unusual approach by not using any GCPs as they are the standard way of improving accuracy but helps demonstrate the point of their value. Quick side note: ground control points (GCPs) are defined as points on the surface of the earth of known location used to geo-reference. We used the most popular DJI Phantom 4 Pro and with no ground control points. The first test sets the baseline of a standard GPS drone. The series of experiments were to really look at the different results achieved when using different acquisition techniques and equipment. ![]() Sitemark has completed over 4000 flights in the past three years resulting in solid experience in acquiring data. Now that we have determined these quality metrics, we can balance our needs with the most cost-efficient solution. It’s important that the stitched drone map perfectly fits on other maps of the same data. Accuracy is the distance/gap from the image point to an existing external reference point such as Google Earth. Over large sites this means photos can start drifting a bit, because you don’t know where photos should exactly lie. When flying with a non-RTK drone, we don’t take the GPS location of the photos into account in the alignment of the photos, because the GPS accuracy is too low. When you have low precision, often it’s a result of a warp in the scan. We’ll talk more about this, but a quick price check shows at $6,500, the RTK (with a premium sensor) is an extra $5000 over the price of the Phantom 4 Pro (basic GPS) at $1,499. Optionally you could invest in a higher quality sensor. At Sitemark, our goal is a gap of about one-centimeter pixel for most applications. So, having a better GSD means that you will be flying lower which requires more time. This is mainly driven by the quality of your sensor, but also the altitude at which you’re flying. The bigger the value of the image GSD, the lower the spatial resolution of the image and therefore, the less visible the details. ![]() The Ground Sampling Distance (GSD) is the distance between two consecutive pixel centers measured on the ground. Quality can be defined as ground sampling distance, precision and accuracy. We’ve chosen three metrics to evaluate quality. If so, let’s agree how to define and measure quality. Is quality and accuracy of output your #1 priority?
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