The scale bar is basically a known distance on the model. For creating scale bars you can use coded targets or set the distance manually between some points on the object. In this article we have used manual approach: measured the lengths of 2 sword blades on the object (see illustration below) and created scale bars using the tools provided in Metashape Professional edition.
While photogrammetry software can add scale to your project with a single scale bar, it is recommended to use at least three scale bars in the project. Two scale bars provide a basic check, and three or more add confidence and statistical reassurance. In this article we describe how to create one scale bar, the steps are to be repeated for each scale bar in the project.
Contents of this article:
How to create a scale bar
To define the end points of a scale bar you need to create two markers on the model. In the Model View set the cursor to the first point and right-click on it to access the context menu. Use Add marker command to create the markers corresponding to the end points of the future scale bar:
We recommend adjusting the position of each marker on at least three images. This will provide better precision for the scale bar. Once the position of the marker is adjusted on the image - the color of the flag will change from blue to green:
Select two markers either in the Reference pane or in the Model view and right-click on them to access the context menu. Use the Create Scale Bar command from the context menu:
The created scale bar will appear in the Scale bars section on the Reference pane: the name of the scale bar corresponds to the names of its' end points (markers). In the example case the scale bar has name point1_point2 as illustrated on the screenshot below:
On the Reference pane in the Distance column enter the length (that had been measured on the object) for the corresponding scale bar. To add the value double-click on the cell in the Distance column and enter the measured distance in designated units. In the Accuracy field enter the accuracy for the distance measurement device that had been used in your case. Please do not forget to use correct units: in the example project, the distances are set in meters.
It is recommended to set scale bar accuracy. You can set this value in the Scale Bars section of the Reference pane: double click on the Accuracy column and write accuracy value (in this case you change accuracy only for one particular scale bar):
Alternatively you can access Reference Setting dialog window by clicking Settings button on the Reference pane and set accuracy for all scale bars in the project.
The Scale bar accuracy that should be specified in the Measurement Accuracy section corresponds to the accuracy of the measured distance. The default value is 0.001 m, it can also be set manually depending on the actual accuracy of measurements for a particular project. In the example project, the accuracy is the same as default value 0.001 m.
Repeat the steps described in the previous section for all distances measured for the project: thus a corresponding number of scale bars will be created (in the example project there are two measured distances and therefore - two scale bars).
After adding or changing information in the Scale bars section of the Reference pane, press Update button to apply the changes. After update the values in Error column will be adjusted.
Click View Estimated button on the Reference pane to see estimated distances (recalculated with the set accuracy):
If on the Reference pane the checkbox next to the scale bar name is enabled, this mans that the scale bar is the control scale bar and it will be used at the optimization stage. If the checkbox is disabled, the scale bar is the check scale bar, it will not be considered on the optimization step instead it can later be used to check the accuracy of the results.
Deleting a scale bar
If the scale bar had been created incorrectly or there is another reason for the scale bar to be deleted, you may follow the next 3 steps and delete the scale bar:
1. Select the scale bar on the Reference pane.
2. Right-click on it and choose Remove Scale Bars command from the context menu.
3. Click OK for the selected scale bar to be deleted.
Scaling of the model
Scale bars allow scaling the model in cases when the georeference data (coordinates of photos and/or markers) is not available in the project. You may check if the scale is applied by performing measurements on the model. Use Ruler to check the length of the elements on the object as illustrated on the screenshot below