Geomatics / Coordinate troubleshooting
Before shifting a drone map in CAD or GIS, compare the coordinate reference, units and height information used by both datasets. A sharp orthomosaic or an RTK fix does not establish that the output is in the receiving project’s reference system.
Preserve the original delivery and diagnose the pattern first. A constant displacement, a scale or rotation difference and an elevation offset lead to different questions; none identifies the correct transformation by appearance alone.
Information checked: 28 October 2026.
In this guide
- First: protect the original data
- Recognize the pattern before choosing a cause
- 1. Compare the horizontal reference
- 2. Check the projection, zone and units
- 3. Separate ellipsoidal height from project elevation
- 4. Review the base, corrections and control points
- Create a coordinate reference record for both datasets
- A hypothetical site handoff
- What to include in a usable delivery
- Frequently asked questions
First: protect the original data
Keep the delivered file, original observations, control-point list and processing report unchanged. Record which software and version opened the data, the import options, the target project and what appears wrong. If you create a corrected version, name it separately and document the transformation.
Then ask the client for its actual requirements: horizontal reference and realization, coordinate epoch where relevant, projection or local grid, vertical datum, units, origin, any site calibration and the expected accuracy or acceptance method. “Canadian coordinates” or “UTM” alone does not specify all of these.
Recognize the pattern before choosing a cause
Scroll within the table to read every column.
| What you see | A question to investigate | What it does not prove |
|---|---|---|
| The whole dataset appears shifted sideways | Are the datum, realization, transformation and project origin consistent? | A constant visual shift does not by itself identify the correct datum. |
| The image is far away or seems enormous | Were degrees, metres or feet interpreted correctly? Was the correct projection and zone selected? | A file that opens successfully is not necessarily using the intended units. |
| The data turns or scales against the drawing | Does the CAD project use a local grid, rotation, scale factor or site calibration? | Rotating it by eye is not an accepted survey transformation. |
| Horizontal placement looks plausible but heights differ | Are the same vertical datum, geoid model, units and height type in use? | RTK status does not prove the height reference is correct. |
| Some areas agree but others distort | Are image geometry, control observations, surface changes and independent checks adequate? | A local mismatch is not automatically a coordinate-system problem. |
These are diagnostic leads, not automatic diagnoses. More than one problem may be present.
1. Compare the horizontal reference
A horizontal coordinate is meaningful only with its reference system and realization. Canada has datasets tied to different versions and epochs, and an engineering project may use a defined local grid rather than a standard projected system.
Compare the source reference recorded during capture and processing with the destination project’s specification. Check the coordinate system assigned on import against the one actually used to create the file. A software label that merely assigns a coordinate system is different from a validated transformation that changes coordinates from one reference to another.
Natural Resources Canada’s GPS·H height-conversion reference describes height conversions using specified reference frames, epochs and geoid models. Use the project’s required reference and an approved transformation; a guessed visual offset is not a substitute.
2. Check the projection, zone and units
A GIS export may be in geographic coordinates while a CAD project expects projected metres, or a CAD file may use a project-specific coordinate grid. Check the projection name, zone, axis order, scale and units in both systems. Also verify whether coordinates were truncated or translated to a local origin for CAD convenience.
A frequent temptation is to drag the orthomosaic into place. That may make one corner appear correct while leaving other areas wrong. Instead, obtain the drawing’s project control or transformation parameters and test the approved result against independent points.
3. Separate ellipsoidal height from project elevation
GNSS observations commonly produce an ellipsoidal height; a project may require elevations in a vertical datum. A geoid model connects these height concepts when applied with compatible references and the appropriate transformation. NRCan describes the relationship as H = h − N, where H is orthometric height, h is ellipsoidal height and N is geoid height. The formula does not tell you which model or reference the project should use; those must be specified and verified. See NRCan’s height reference explanation.
Ask whether the processing and receiving systems used the same vertical datum and geoid model, whether conversion was applied once or twice, and whether the height is expressed in metres or feet. Do not add a constant elevation offset solely because the surface looks more convincing afterward.
4. Review the base, corrections and control points
An RTK fixed solution can improve positioning, but it does not establish that the base coordinate, coordinate frame or output transformation is correct. Review the correction source, base position, coordinate reference, solution status, antenna or pole height, and the point descriptions. If data came from multiple visits, compare the setup records across dates.
Ground-control points may constrain or adjust a reconstruction. Independent checkpoints test the resulting dataset. A point used to fit the model is not independent evidence of its accuracy in the same processing run. Compare checkpoints distributed across the project and its elevation range using a method and tolerance agreed with the survey lead.
For field-mode and handoff considerations, read Base, Rover or Relay? How to Use DJI D-RTK 3. The D-RTK 3 product page is a starting point for a compatible configuration; confirm the aircraft, mode, firmware and correction workflow before purchasing.
Record the correction station’s position, reference system and setup for your project.
Image credit: DJI.
Create a coordinate reference record for both datasets
Complete the same fields for the source output and the receiving drawing or GIS project. Ask the project survey lead to resolve missing or contradictory entries before approving a conversion.
Coordinate reference checklist
- Horizontal reference
- Frame or datum, realization and epoch where applicable.
- Source and destination must be identified, not inferred from a filename.
- Projection and units
- Projection, zone, axis order, coordinate units and any local origin.
- “UTM” alone leaves important choices unspecified.
- Height reference
- Height type, vertical datum, geoid model, units and epoch where relevant.
- Record whether a height conversion has already been applied.
- Project adjustment
- Approved local grid, scale, rotation or site calibration.
- Keep the actual transformation parameters and their source.
- Verification
- Independent check locations, results and acceptance requirement.
- Verify the corrected version; retain the original and transformation log.
Assigning a reference tells the software how to interpret existing numbers. Transforming coordinates calculates new numbers in another reference. Assigning the desired destination label to source numbers does not perform the conversion.
For example, a hypothetical ellipsoidal height of 110 m and geoid height of 30 m gives an orthometric height of 80 m using H = h − N. These values illustrate the calculation; they are not a correction for a real Canadian site. The appropriate reference, model and transformation still need to be established for the project.
A hypothetical site handoff
A construction team receives an orthomosaic for a site drawing. The image lines up with a building at one end but differs near the other. The team first checks units, projection and the drawing’s local-grid documentation. It then compares independent control observations across the site and asks the survey lead to inspect both the transformation and reconstruction quality.
Have the responsible professional review the project information and checkpoint results before choosing a transformation or reporting accuracy.
What to include in a usable delivery
Alongside the agreed files, provide a short handoff sheet with:
- Horizontal reference, realization, epoch if applicable, projection or local grid and units.
- Vertical datum, height type, geoid model and units.
- Capture date, processing software and version, and transformations applied.
- Control and independent checkpoint roles, methods, results and project acceptance criteria.
- Coverage limits, excluded areas, known changes and outstanding questions.
The municipal drone GIS workflow guide expands this into a receiving-team and data-ownership process. DJI Terra is one processing option; confirm the licence, output format, coordinate transformation and workstation requirements for your specific workflow.
Frequently asked questions
Does RTK mean I can skip independent checkpoints?
Not automatically. RTK describes a positioning method; the project’s quality requirements determine how the finished deliverable is tested. Independent checks are distinct from points used to fit a model.
Can I fix the map by moving it until it looks right?
That can conceal the cause and may introduce errors elsewhere. Preserve the original and use the transformation or site calibration approved for the project, then verify the result against independent evidence.
Why does the map fit horizontally but not vertically?
Investigate height type, vertical datum, geoid model, units and any conversion applied during processing or import. Similar-looking horizontal coordinates do not establish equivalent heights.
Do I need a professional surveyor?
It depends on the use, jurisdiction and project requirements. For legal boundaries, regulated work, engineering acceptance or other consequential decisions, involve the appropriately qualified professional before relying on the result.
Prepare a mapping support request
Gather the original files, mismatch description, correction source, processing settings and the receiving project specification. That information makes an equipment or workflow discussion useful.
