This troubleshooting guide helps you identify errors you might come across in your photogrammetry and offers solutions for those errors. If your project is set up properly, Terrain Creator will create accurate survey-grade results. If your results aren't accurate, this guide provides insight on errors that may have been introduced during the photogrammetry process.
Overview
Recognizing Errors with My Data
Problems you may come across can be identified in three main areas:
- Ground Point Markers do not center on your screen during the marking process.
- If you see this happening, we recommend that you do not process your photogrammetry.
- High errors in your Z elevation.
- Horizontal errors.
Once you recognize that there are errors in your data, you can find solutions to those errors in the Table: Situational Errors and Solutions table of this troubleshooting guide.
Recognizing Errors Before You Process Your Photogrammetry
You can recognize problems in your data before you process your photogrammetry. This section explains how you can.
The indication that an error is present in your data is when you are marking your ground control points. When you initially start marking your ground control points, you will not be able to notice any potential errors. You just go through the first marking cycle (marking each ground control point 3 times) and initially, the ground markers may not align in the center of your ground marking window. This is not something to worry about as you cannot be sure yet whether there is a problem or not.
On the second cycle through marking ground control points, if the ground markers still aren't getting any closer to the center of the ground marking window, you will undoubtedly know that you have a problem with your measurements because Terrain Creator is having a difficult time triangulating the position of the ground control point in the photo, in relation to the ground marker.
We strongly recommend you stop working on your data at this time and review our table for error cases and solutions before continuing on.
You might want to try and process your data anyway, despite Terrain Creator not being able to home in on the ground control points. And if you still want to take the time to process your data at this point, you will see the errors and inaccuracies are confirmed after your data is processed.
Incorrect: Example of the second wave ground markings not centering in the ground marker window
Correct: Example of the second wave ground marking properly centering in the ground marker window
Recognizing Errors After You Process Your Photogrammetry
It is much easier to recognize errors in your data after you've processed your photogrammetry. You can assess errors in the Terrain Creator app using the included tables for your Ground Control Points (GCPs) and by visually looking at the location of GCPs over your ground markers.
High errors to Z elevation shown in Terrain Creator's Ground Points table
Visual example of a Z elevation error for Ground Control Point 20 @ -1.663 ft
Visual example of a Horizontal error shown for Ground Control Point 40 with both X and Y being off-center
Table: Situational Errors and Solutions
This table can help identify errors that are in your dataset and the solutions you can use to correct those errors. Any errors you come across might be caused from the following situations:
| Case Error | Solutions |
|---|---|
Case #1: Coordinate System with a Scale Factor You want to work in a coordinate system that has a scale factor applied. Consequently, your ground points are measured in ground coordinates that are properly scaled in relation to the grid coordinates, but your camera locations don't have the same scale factor applied and are still in grid coordinates. | Scale Factoring Solution - Case #1 Currently, coordinate systems with scale factors are not supported, but this functionality is on the road map of things to come. We currently only support EPSG coordinate systems. |
Case #2: Relative RTK with a Single Local Base Station You have set up a local base station with a 'here position' (aka base coordinate) that is determined by the base station itself, which doesn't actually have a real-world correction to a Network Control Station. This makes the base station's accuracy an estimated 1-5 meters off from the real-world correction position. This means the drone flight camera positions have a higher accuracy when compared to the base station, but still not to the level of real-world accuracy your Ground Control Points are measured in. | Relative RTK Solution, Single Base Station - Case #2 This case is automatically detected and corrected from the Virtual Surveyor 9.5 update and up. Upgrade to the latest version of the Terrain Creator app. |
Case #3: Relative RTK with Multiple Local Base Stations You have completed multiple drone flights, maybe on different days. For every flight, you have set up your base station at a different location or had a different 'here position' measured. You might also have used a different base station from the one you set up as the local base station at the same position but applied a different elevation. For example, you might have overlooked or not compensated for the antenna height on one station, while you did on the other. | Relative RTK Solution, Multiple Base Stations - Case #3 Process the drone flights that were measured with different base station locations separately. Then in the Virtual Surveyor app, use the merge and clip features to put the different orthophotos and elevation models together as a mosaic. |
Case #4: False RTK Your camera accuracy tags included with your drone photos reflect high precision, but in reality, they are not actually precise. For example, you have loaded photos into Terrain Creator in a .jpg format, which contains EXIF coordinates that are measured to a base station that does not maintain an absolute or fixed position:  | False RTK Solution - Case #4 See the solution for Case #5 |
Case #5: All Other Situational Errors Other cases may include, but are not limited to: - You find out later that the base station stopped working during the drone flight. - Cannot get a solid connection to the Network Control Station. - You used standard GPS for your camera positions, but you still cannot get a reliable connection to the base station. - Etc. | All Other Situational Solutions - Case #5 Create your project and do the following: You will need to create a .csv file with your camera positions and add the XY and Z Accuracy tags with an accuracy tolerance of 10 m or more, e.g.:  With an appropriately tagged .csv file, you can add the file to your project and start marking and processing your data as a GPS Project because you have now allowed more freedom for the camera positions to adjust. With this solution, you do remove some advantages of an RTK project. If you do this, it is essential that you have enough ground control points on your project site in order for Terrain Creator to make accurate corrections (triangulations) between your JPEG camera positions and their relation to the ground control points. |
If your case falls outside of the solutions we have here, or if you have any questions, please contact us. We'll get back to you as soon as we can.