Rock Mapper Knowledge Base - Step 3: Georeferencing

Intro

Rock Mapper is able to georeference rock face photographs as well as LiDAR scans in 3D space using:

A scale bar (to scale the photograph; LiDAR scans are already correctly sized/scaled)
A face heading value (= orientation of the face which the face photograph and 3D LiDAR scan are aligned to)
A reference point (aka Point) on the face OR a drive direction laser station (aka laser station) + distance. More details in this article.

ℹ️ In case there is no surveyed known/point or laser station available at the time of mapping, you can later add these info and update the rock face with it.

Workflow

👉 A) Georeferencing a face photograph

The app is able to place a 2D photograph in 3D space so it can be directly imported into 3D modelling software packages. Moreover, all annotations associated with the photograph (such as contacts, structures, POIs and Scribbles) as well as all sampling data are also made available in a fully georeferenced format based on the orientation and position of the photograph.

The georeferencing toolbar in the lower left corner is used to add all georeferencing information. Simply work your way from left to right and follow the on-screen instructions.

ℹ️ Please note that there is a difference between tapping the number / name or the icon in front of each field. By tapping the number / name you will be able to edit the values. By tapping the icon in front of the number/name, you will be able perform a certain command such as:

Setting a scale bar / face width: Draw a scale bar to set the face width (will be used to scale the photograph)
Measure the face heading: Use the iPad's built-in magnetic compass to measure the orientation of the face (will be used to orient the georeferenced face photograph). Please note that this value is automatically captured when taking a picture of the face with Rock Mapper.
Select the reference point: Select the location of the reference point on the photograph or LiDAR scan (will be used to position it in 3D space)

1. Define face outline

Tap the first icon from left and draw the face outline onto the photograph. This outline is subsequently used as the outer boundary for the sampling grid on the sampling page and is also used to calculate the face area. The outline is saved in the FaceID_outline.csv / dxf file which is included in the Rock Face ZIP file.

2. Add a scale bar

We recommend to use the face width for scale. Therefore take a laser disto to measure from the left wall to the right wall. Then tap the scale bar icon and draw a line from the left side to the right side and enter the measured distance. A scale bar is then added and the photograph scaled to match the measured length.

3. Add face heading (= orientation of face)

Tap compass icon to measure the orientation with the built in magnetic compass or tap the field to manually enter a known heading value. If you have taken the picture with the built in camera then the face heading value is automatically captured and copied over (more details here).

4. Pick reference point

Tap the cross hair icon and then select the projected laser point on the face (or any other known point). You'll be prompted to enter the distance from the projected point on the rock face to the laser station (measure this distance with a laser disto). This value is used by Rock Mapper to then calculate the 3D coordinates of the projected point on the rock face. It's using the X/Y/Z, azi and plunge values that are associated with the laser station and the entered distance value to calculate the 3D coordinates of the projected laser point on the rock face.

5. Select laser station from list

Tap on the Laser Station list to select the reference station. Read this article on how to add a laser station to this list (for fresh/new installations this list will be empty).

ℹ️ The face photograph and the LiDAR scan share the same georeferencing information. The georeferencing toolbar in 2D View and 3D View are very similar. Only the scale bar button / field is missing in the 3D View because a LiDAR scan is automatically scaled when it is acquired (lasers can measure distance). Plus there is a Rotate-to-Align button for LiDAR scans.

👉 B) Georeferencing a LiDAR scan

The georeferencing toolbar in the 3D View is located in the lower left corner and is used to register the LiDAR scan in 3D space. Any existing georeferencing information from the 2D View / face photograph will be automatically inherited. The workflow below assumes that you have already a georeferenced photograph in the project.

1. Rotate LiDAR scan to align it with the face heading (blue arrow)

Tap alignment button in lower left corner (first from left in georeferencing toolbar in 3D View)

You'll then see guide lines that are perpendicularly oriented to the heading value. Use slider to rotate the point cloud until it aligns with the guide lines and the blue heading arrow. The rock face should be parallel to the guide lines on the ground and perpendicular to the blue arrow.

Before rotation. Not aligned with the blue heading arrow pointing towards 348 (North)

After rotation. Rock face is aligned with guide lines and perpendicular to blue heading arrow.

After the LiDAR scan is aligned, tap the tick mark icon in lower left corner to lock in the new orientation.

2. Pick position of reference point / projected laser point

Same as for the rock face photograph, a single point is used to position the LiDAR scan in 3D space. To select the location of it, tap the reference point icon in the georeferencing toolbar and then pick its location on the LiDAR scan.

Reference point shown as pink sphere in 3D View after it has been picked.

ℹ️ Rock Mapper's LiDAR scans are automatically scaled and levelled (by measuring the gravity vector while scanning). Therefore only a single reference point with known coordinates and the orientation around a vertical axis to match the face heading are necessary to fully georeference the LiDAR scan in 3D space.