Rock Mapper Knowledge Base - Step 3: Georeferencing

Intro

Rock Mapper is able to georeference rock face photographs and 3D LiDAR scans using following options:
A) One Drive Direction Laser Station + Face Heading
B) One Survey Point + Face Heading

C)* Two Reference Points (such as Survey Points, Laser Stations and picked points on other LiDAR Scans OR a mix of it e.g., one Survey Point and one Laser Station).

*Only available for 3D LiDAR Mesh Scans

ℹ️ In case there is no survey point or laser station available at the time of mapping, users can add these at a later stage and all rock face data will then automatically update.

Workflow

👉 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.

For georeferencing of rock face photographs, Rock Mapper uses:

A scale bar (to scale the photograph)
A face heading (= orientation of the face which the face photograph is aligned to)
A Survey Point on the face OR a drive direction laser station (Laser Station) + distance. More details in this article.

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)

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. The small indicator light will change from red to green as soon as the rock face is fully georeferenced.

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 OR use Apple's Measure app that comes pre-installed on any iPad Pro. 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 accordingly.

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 rock face photograph with the Rock Mapper app, then the face heading value has already been captured (using the iPad's built in magnetic compass) and it is automatically copied over (more details here).

4. Pick reference point

Tap the cross hair icon and then select the projected laser point OR survey point on the face. For type laser station, 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 or survey point from list

Tap on the Laser Station / Survey Point list to select the reference laser station / point. Read this article on how to add a laser station and survey points to this list. For 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 and an option to use 2 Points for georeferencing.

👉 B) Georeferencing a LiDAR scan

The georeferencing toolbar is located in the lower left corner and is used to register the LiDAR scan in 3D space. Any existing georeferencing information from the picture / face photograph will be automatically inherited and used for the LiDAR scan as well. 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

The face heading has already been captured when taking the face photograph (in example below it's N 348). If you want to measure the face heading again, then tap the icon in front of the face heading value.

Now, tap the 'Rotate to Align' button in lower left corner (first from left in georeferencing toolbar)

You'll then see guide lines that are oriented perpendicular to the heading value (blue heading arrow). Use slider to rotate the 3D scan until it aligns with the guide lines. For fine adjustment use the plus and minus stepper buttons. The rock face should align with the guide lines on the ground and be facing the same way as 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, you can use a single point 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.

Rock Mapper version 3.3 introduced the option to georeference 3D LiDAR mesh scans using 2 Points instead of 1 Point + Face Heading. Simple select the 2 Points option in the georeferencing toolbar to enable it.

ℹ️ Rock Mapper's LiDAR scans are automatically scaled and levelled (by measuring the gravity vector while scanning). Therefore only a single reference point and the orientation around a vertical axis to match the face heading OR 2 reference points (NOT 3) are required to fully georeference the LiDAR scan in 3D space.

With two points the scan can be locked into it's position AND orientation (without the need of the 'Rotate to Align' step). Moreover, LiDAR scans of subsequent rock faces can be georeferenced based on shared points on the previous rock face. In other words, users are able to accurately 'daisy-chain' rock faces without the need of survey control. We suggest to follow this ‘daisy-chaining’ workflow:

1) For the initial rock face, use 2 proper Survey Points for georeferencing, one on the left and one on the right wall of the scan.

2) Spray paint a left wall point and right wall point before blasting. These points don't have to be surveyed and can be positioned anywhere on the left and right wall (we recommend around 0.5 - 1m back from the rock face at eye level). These two points will be used as reference points for the subsequent rock face (after blasting).

3) After blasting, make sure to scan the walls of the new rock face a little further back to include the spray painted points on the left wall and right wall of the previous rock face. These two points are then used as reference for the new scan by choosing the Pick on Scan option in the georeferencing window. You can then directly pick the matching points on the 3D scan of the previous rock face in a convenient split screen view.

Please note that you can only select fully georeferenced scans to be used as a reference (as indicated by the icon in the scan list, see screenshot below).

ℹ️ After around 5-10 rock faces, we suggest to use proper survey control points again because georeferencing inaccuracies (due to manually picking points) might add up.