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GCP Basics

What is a Ground Control Point (GCP)?

A Ground Control Point (GCP) is a small point on the ground with a known coordinate. Ground Control Points can be marked by just about anything, but the most common markers are stakes, nails, posts, and even concrete monuments in some cases.

Our Ground Control Points are specially made for aerial mapping with drones and other aircraft. In reality, our Ground Control Points would be more accurately referred to as "aerial targets," but most people use the term "GCP" for the physical targets as well as the known coordinates.

The reason that you need our GCP targets is that most of the markers (nails, stakes, posts) would be difficult or impossible to find from aerial images, especially when some images can be taken hundreds (or even thousands) of feet in the air.

Using our targets, the user (or the software in some cases) is able to accurately identify the Ground Control Point by selecting the center of the target.

Do I need Ground Control Points for my map?

Having GCPs positioned across the mapped area allows for reconstructed maps to be scaled accurately. One way to think about it is that GCPs pull the image data from up in the sky (where the drone took the photos) and anchors the photos (and map) to the surface of the earth. The GCPs will also allow the software to stretch the map so that each point in the map is positioned at the exact coordinate.

Maps created without GCPs rely primarily on the positioning data within the capture device (typically the GPS a drone). As a result, the maps created without GCPs are not directly tied to points on the ground, which can result in inaccuracies in the mapping data.

How many GCPs do I need?

This is a highly-debated topic, and the most likely answer is "it depends". For most maps, having 5 GCPs is sufficient in most cases. The standard layout of the GCPs across a rectangular mapping area should be 1 GCP at each corner (4 in total) and 1 GCP in the center of the map. Having a GCP in the center of the map will increase the accuracy of the elevation across the map (Z direction). Each additional GCP above 5 GCPs will likely increase your accuracy to some extent, but the increase in accuracy may be so small that it's irrelevant. For larger projects, additional GCPs are recommended as well as for projects that are not rectangular in shape.

Are GCPs the same as checkpoints?

No - GCPs are provided to the software to accurately position and scale the map. Checkpoints are similar to GCPs in that they are known coordinates inside of the mapped area, but these coordinates are used to "check" the accuracy of the map after the GCPs have been used to create the map.

Checkpoints are important because they allow you to make sure that the error is similar across the map. Depending on the numerical weights given to GCP coordinates, some software programs will adjust the map to "fit" the GCP coordinates, which will artificially inflate the accuracy of the map. In other words, the software is doing its best to fit the map to the GCPs at the cost of the other areas of the map which can cause areas without GCPs to be wildly inaccurate. We recommend estabilishing checkpoints across the map to ensure that the error figures at the GCPs are consistent across the map.

Mapping Basics

How does mapping with drones work?

In recent years, mapping large areas of land has become more and more popular due to the advancements in unmanned aerial vehicles (UAVs), otherwise known as drones. In order to map large areas at low altitudes, these drones take hundreds of photos of the ground, with each photo overlapping some with the photo next to it. These photos are then processed using software and "stitched" back together based on the part of each photo that overlaps. The process of stiching the photos back together is known as "photogrammetry".

What do I need to get started mapping?

To get started with your first aerial map, you will need 3 things:

1. A Drone

Most modern drones are capable of mapping. The DJI Phantom 4 Pro has been the workhorse drone for mapping since its release due its mechanical shutter and long flight time. More recently, the DJI Mavic Pro 3 Enterprise has shown promise as a legitimate replacement for the Phantom 4 Pro. A wide variety of custom options exist for enterprise drone mapping operations.

2. A Computer

Based on the software that you select, your computer may be a piece of your mapping workflow. With some maps reaching thousands of photos, your specs are important to enable fast processing times. If you do not have access to a computer with the specifications needed to reconstruct your map, there are cloud based services available for processing your data.

3. Software

There are a wide variety of photogrammetry options available including Pix4D, AgiSoft Metashape, DroneDeploy, and RealityCapture. If you are looking for a turnkey solution for your mapping needs, Pix4D or Agisoft Metashape are the two industry standard software offerings.  If you are just getting started, DroneDeploy is a good choice to learn the basis of mapping without needing a powerful computer due to their affordable cloud processing options. Other aerial photogrammetry options include RealityCapture, Bentley ContextCapture, Autodesk ReCap, and many other open-source options that deliver good results if you're willing to familiarize yourself with the software.

What about survey equipment and GCPs?

If you're reading this FAQ, you are probably thinking about making your first aerial map with a drone. In order to get survey-level accuracy, you will need to invest in survey equipment. If you are just looking to get started in aerial mapping, we recommend making a few maps before purchasing surveying equipment (including GCPs). If you want to get the experience of placing GCPs without investing in survey equipment, just drop a pin in Google Maps at the location where you place your GCPs so you can get the idea of how they work. Import your pin locations into Google Earth. Then, you can use these pins to manually scale your map so that the pins that you placed are positioned in the same place as the GCPs that you see on your map. Again, this will not be anywhere close to a survey grade map, but it will give you an idea of the general process. Who knows, you might be picking up some survey gear next!

Do I need a license to fly a drone?

If you only plan to fly the drone recreationally, you do not need a drone license. However, there is a short knowledge and safety test that you will need to complete before flying your drone. The test is free and can be taken online at your convenience.

If you plan to do commercial work (aka get paid for what you do) with your drone, you will need a Part 107 license, which involves an in-person, paid test that requires some knowledge of general aviation principles.

The link below gives a good overview of the regulations for those who are interested in learning more.

Do I need to be a licensed surveyor to make maps?

Great question - and, unfortunately, the answer is "it depends".

To be legally signed off as a map that references boundaries, most states require that the map have a licensed surveyor's signature. However, this does not mean that the map was created by the surveyor, so in some cases, a drone pilot will fly over the area with their drone and create the map. Then, a surveyor will check to ensure everything is accurate before signing the map.

Oftentimes, surveyors will place the ground control points in advance, and then someone else will come and fly the mission (which was part of why we made the passthrough GCP).

In some cases, the goal of the drone map is not for boundary lines or legal purposes, so someone without a surveyor's license could do the entire job without requiring a signature from a surveyor. For example, a construction company may want to measure a stockpile of material or a developer may want someone to create a timelapse of a large development project. Neither of these examples would likely require a surveyor's license.

So, the best answer is that it depends. Hopefully, this answer has at least given you an overview of the mapping landscape. This is not legal advice, and we would recommend reading about the guidelines in your own state based on what type of work you want to focus on.

Deciding on a GCP

GCP Size - 24"x24" vs 48"x48"

We currently offer 2 sizes of Ground Control Points (GCPs) - 24"x24" and 48"x48". For most drone mapping operations, a 24" GCP will be sufficient at altitudes of up to 400' (which is the legal limit in the USA). The 48"x48" GCPs are easier to locate during prost processing and can be especially useful with LiDAR missions where the point cloud may be sparse in some areas.

GCP Pattern - Checkerboard vs Iron Cross

We offer 2 patterns of GCPs - checkerboard and iron cross (also known as a harlequin cross). The checkerboard pattern is the most common aerial target pattern. The iron cross creates a longer leading edge to the center point of the target, which some claim leads to increased accuracy when identifying the center point of the GCP.

*Please review your software program to see which pattern is preferred. Some software is optimized to identify certain GCP patterns.

GCP Center Type - Center Eyelet vs Center Passthrough

We developed two center types for out GCPs - the center eyelet GCP and the center passthrough GCP.

Center Eyelet GCP

Allows the user to easily locate the center of the target without causing the target to tear; also allows for survey nails to be placed through the center eyelet; ideal for placing GCPs as you record the GCP coordinates

Center Passthrough GCP

Allows the user to use pre-established survey stakes without removing the stakes from the ground; ideal for placing GCPs after GCP coordinates have already been established and marked with survey stakes

GCP Color - White and Black vs Grey and Black

We currently offer 2 color patterns for our GCPs - white and black or grey and black. White and black is the most common type of aerial target due to the high contrast that the colors provide with the ground. However, a light grey has been shown to reduce overexposure of the aerial targets, especially on sunny days, allowing the counterpoint to be more easily identified.

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