Accurate georeferencing is essential for creating reliable, high-quality data products. Ground control points (GCPs) play a crucial role in ensuring that your drone imagery is precisely aligned with real-world coordinates.

It would be really nice if they were really easy to set up, wouldn’t it? Sometimes it is. But setting them up can be challenging in various land and marine environments. In this blog post, we’ll explore what GCPs are, how to establish them in different settings, and alternative methods for when precise positioning isn’t possible.

What are Ground Control Points?

GCPs are visible markers placed at known locations within your mapping area, serving as reference points for georeferencing drone imagery. By identifying the coordinates of these markers in your images and comparing them to their real-world positions, you can accurately align and scale your drone maps to the desired coordinate system, such as WGS84 or a local grid.

GCPs can be natural features, such as rock outcroppings or tree stumps, or artificial markers, like painted targets or survey flags. The key is that they must be clearly visible in your drone imagery and have precisely measured coordinates.

Setting Up GCPs in Different Land Environments

The process of establishing GCPs varies depending on the terrain and accessibility of your mapping area. Here are some tips for setting up GCPs in common land environments:

  1. Flat terrain: In open, flat areas like fields or plains, you can use large, brightly colored markers like plastic tarps or painted plywood sheets. Place them in a grid pattern across your mapping area, ensuring that they are evenly distributed and visible from your drone’s maximum flight altitude. Use a survey-grade GPS or total station to measure the coordinates of each marker.
  2. Mountainous terrain: In rugged, mountainous areas, look for natural features like rock outcroppings or boulders that are visible from the air. Mark these features with spray paint or survey flags to make them more prominent. If possible, use a survey-grade GPS with real-time kinematic (RTK) corrections to measure their coordinates accurately, as this will help account for the challenges of satellite visibility in steep terrain.
  3. Desert environments: In desert settings, use high-contrast markers like black and white checkered targets or bright orange survey flags. Secure them to the ground with stakes or rocks to prevent them from being blown away by the wind. Be mindful of the shifting nature of sand dunes and choose stable locations for your GCPs.
  4. Coastal areas: Along coastlines, use natural features like large rocks or man-made structures like piers or seawalls as GCPs. If using artificial markers, ensure that they are securely anchored to prevent them from being washed away by waves or tides. Consider the tidal range when measuring GCP coordinates, and aim to collect drone imagery during the same tidal phase to maintain consistency.

Establishing GCPs in Marine Environments

Setting up GCPs in marine environments presents unique challenges, as water bodies lack stable, permanent features that can serve as reference points. Here are some strategies for establishing GCPs in different marine settings:

  1. Ocean: In open ocean environments, use floating markers like buoys or surface vessels equipped with high-precision GPS receivers. Ensure that the markers are large enough to be visible in your drone imagery and are anchored securely to prevent drift. Collect drone imagery and GPS measurements simultaneously to minimize the impact of marker movement.
  2. Rivers: In riverine environments, use natural features like large boulders or man-made structures like bridges or dams as GCPs. If using artificial markers, choose locations along the riverbank that are stable and accessible. Be mindful of changes in water level and flow when measuring GCP coordinates and collecting drone imagery.
  3. Lakes: In lake environments, use a combination of shoreline features and floating markers as GCPs. For shoreline features, choose stable, visible points like rock outcroppings or piers. For floating markers, use buoys or anchored platforms equipped with GPS receivers. As with ocean environments, collect drone imagery and GPS measurements simultaneously to account for marker drift.

When Precise Positioning Isn’t Possible

In some situations, precise positioning of GCPs may not be possible due to limitations in GPS accuracy or the lack of stable, permanent terrain features. In these cases, consider the following alternative methods:

  1. Visual alignment: If your mapping area has distinct, recognizable features like roads, buildings, or natural landmarks, you can use these as visual references for aligning your drone imagery. While this method may not provide the same level of absolute accuracy as GCPs, it can still yield useful relative accuracy for many applications.
  2. Relative scaling: If you have access to high-resolution satellite imagery or existing maps of your study area, you can use these as a reference for scaling and aligning your drone imagery. By matching visible features between your drone maps and the reference data, you can achieve a reasonable level of relative accuracy, even without precise GCPs.
  3. Post-processing adjustment: In some cases, you may need to rely on post-processing techniques to improve the alignment and accuracy of your drone maps. This can involve using software tools to manually adjust the position and scale of your imagery based on known reference points or overlapping features between adjacent images.

The key to successful ground control in drone mapping is to be adaptable and creative in your approach. By understanding the challenges of different environments and being prepared with a range of strategies, you can ensure that your drone data is accurately georeferenced and suitable for your environmental research needs.

When precise positioning isn’t possible, focus on achieving the best relative accuracy you can and be transparent about the limitations of your data when sharing or publishing your results. By continuously refining your methods and learning from each mapping mission, you’ll develop the skills and knowledge needed to tackle even the most challenging ground control scenarios, all while contributing valuable insights to our understanding of the natural world.