Understanding Ground Control Points (GCPs) and Maintenance

Learn what GCPs are, how they're placed, how to upload coordinates, and how maintenance requests work.

Written by Raymond Fuorry
March 31, 2026

Ground Control Points (GCPs) are physical markers placed on your construction site that ensure your drone scans are accurately georeferenced. This article explains what GCPs are, how they work, and how to maintain them.

What are Ground Control Points?

Ground Control Points (GCPs) are physical markers — typically painted crosses on concrete or tarp-based targets on soil — used by the TraceAir GIS team to reference elevation and ensure scan accuracy. GCPs provide known coordinates (Easting, Northing, and Elevation) that the photogrammetric processing uses to align the drone images with real-world positions.

Important: GCPs are different from existing survey control points (survey monuments used by engineers for design). GCPs are placed specifically for drone scan processing and must remain undisturbed between scans.

How are GCPs set up?

GCPs can be placed in two ways:

• By the customer's surveyor: Your surveyor places the markers and provides the exact coordinates to TraceAir.
• By TraceAir as a service: TraceAir offers a GCP maintenance contract where the pilot checks and replaces GCPs on each site visit.

After the initial preliminary flight (an internal TraceAir flyover used to assess the site), TraceAir sends a GCP location request email with recommended placement positions. Your surveyor then places flight markers at the recommended locations, measures the exact coordinates, and provides the data back to you as a CSV, XLSX, or PDF file for upload.

Where should GCPs be placed?

The GIS team recommends placing GCPs on pavement or concrete whenever possible, because markers on earthen surfaces are more likely to be disturbed by grading activity. Additional guidelines include:

• GCPs require a flat 6-by-6-foot area per marker.
• Avoid areas with heavy grading work, stockpiles, slopes, or tall vegetation.
• Do not place GCPs near tall objects (walls, buildings, rocks) or under trees and power lines.
• If optimal locations are difficult to access, stay as close as possible (within a 30-foot radius) to the recommended positions.
• The number of GCPs depends on site size.

For detailed marker placement instructions, including materials needed and surface-specific techniques, refer to How to place and maintain flight markers (GCPs).

How to upload GCP coordinates

You can submit GCP coordinate files in two ways:

1. Upload through the Project Portal: For new projects, upload through the Flight Markers card in the Project Setup tab. For ongoing projects, navigate to the Ongoing Projects tab, select your project, and upload the file.

The GIS team accepts GCP coordinates in CSV, XLSX, or PDF format. Coordinates should be recorded in the project's coordinate system, matching your design data.

Note: TraceAir recommends providing a full survey report of the flight marker measurements, with three measurements per point (reinitializing the GNSS receiver for each measurement), along with a photo of each GCP.

How do GCP requests work in Ongoing Projects?

When the GIS team determines that new GCPs need to be placed or existing ones maintained (repaired, cleaned up, or replaced), a GCP request banner appears at the top of your project detail view in the Ongoing Projects tab. The banner shows the request date, a summary (e.g., "New: 6, Clean Up: 1, Total: 7"), and a deadline (e.g., "Please ask the surveyor to place these markers before the next scan on [date]").

From the GCP request banner, you can:

• Click View request to open an interactive map showing all GCP locations. The map displays Active markers (existing, working GCPs) and Requested markers (new or maintenance-needed). Each marker is labeled on the map (e.g., "Requested_1," "200022") and listed in a sidebar with categories: New Markers (new GCPs to be placed) and Clean Up / Repaint (existing markers needing maintenance).
• Click CSV (EPSG:[code]) or CSV (WGS84) to download the recommended GCP coordinates in your project's coordinate system or in WGS84 format. Share these coordinates with your surveyor.
• Click Placement Guide to view the detailed Flight Marker Guide with placement instructions.
• Click Upload GCP coordinates to open the upload dialog once the surveyor has placed the markers and provided measurements.

The Upload GCP coordinates dialog asks you to upload a full survey report of the GCP measurements (downloaded from the rover). The preferred format is CSV. You can add an optional comment before clicking Send.

How does GCP maintenance work?

The GIS team reviews GCPs after every scan. If a marker appears disturbed, damaged, or undetectable, an automated email request is sent to the users listed as Point of Contact for the project. The request also appears as a GCP request banner in the Ongoing Projects tab, with a link to an interactive map where you can view the GCP locations and their status.

GCP maintenance statuses include:

• Replace — Damaged: The marker is almost destroyed but still visible. It needs to be restored.
• Replace — Undetectable: The marker is not recognizable in the drone images. It needs to be made visible from the air.
• Replace — Faded: The paint on the marker is barely visible and needs to be refreshed.
• Clean Up — Obstructed: Objects (rubbish, materials) are placed over the marker and need to be removed.
• Clean Up — Covered: The marker is covered by dirt, grass, or snow and needs to be cleaned.
• Check Condition: The marker is visible and usable, but significant time has passed since the last check.
• Send Coordinates: TraceAir can see the marker in the scan but has not received the coordinates.
• Remeasure: The coordinates need to be re-measured in the global coordinate system.

Important: When maintaining a GCP, do not move or touch the marker itself. Even the smallest change in its position will make it unusable for scan processing. Only clean around it or refresh the paint.

What is the QC/GCP Email tab?

The QC/GCP Email tab in the Project Portal lets you manage who receives Quality Control and GCP notification emails. You can search by project and by recipient, and assign each person to receive QC notifications only, GCP notifications only, both, or remove them from these emails entirely.

Frequently asked questions

What is the difference between GCPs and survey control points?

GCPs are physical markers placed specifically for drone scan processing and georeferencing. Survey control points (also called monuments or benchmarks) are permanent reference points placed by engineers for site design and construction layout. They serve different purposes, though both provide known coordinates.

Can I view the GCP locations on a map?

Yes. When you receive a GCP request email, it includes a link to an interactive map where you can view the requested GCP locations and their current status.

What is the Coordinate Reference System (CRS) form?

Before your first flight, TraceAir requires a Coordinate Reference System (CRS) to be defined — this is what unlocks TraceAir processing for your project. Without the CRS, measurements will not be available. The CRS form requires information about the Plane CRS (including units and whether it is published or assumed) and the Vertical CRS. You can complete the form yourself by clicking Open CRS form in the Coordinate System card, or forward it to your engineer or surveyor by entering their email address and clicking Send CRS form. If CRS information is unclear, TraceAir recommends consulting a surveyor or engineer. A control plan (PDF and CSV, or DWG and CSV) showing control point locations is also required if TraceAir is placing the GCPs. The Coordinate System card applies to Gold and Platinum scan tiers only.

Who should I contact about GCP maintenance costs?

For questions about GCP maintenance contracts or pricing, contact your TraceAir Account Manager at usam@traceair.net or your Customer Success specialist at uscs@traceair.net.