Georeferencer User Guide - Plan Creator
1 In Plan Creator, move the map to the location of the subjects that require to be georeferenced.
2 To open the georeference tool, click on the georeferencing tool icon from the toolbar.
3 This will open the georeferencing tool window. The first task is to upload the deed plan image that requires to be georeferenced. This file should either be in jpeg, jpg, png, or tiff format. The file can either be dragged and dropped into the upload box or selected from File Explorer.
The file name will now appear, and if this is the correct plan to be georeferenced, click on the Upload button to upload the file. If it is the wrong plan click on Remove and then select the correct file.
4 The plan to be georeferenced will now appear on the left-hand side of the screen, with the OS map on the right-hand side. The plan to be georeferenced can be moved around the screen independently of the OS map by dragging the image on the screen using the mouse, and the zoom level altered by using the scroll wheel or the + and – buttons. If required, the image can be rotated using ALT + SHIFT + left mouse click and drag the mouse.
5 The first step is to identify the control points to be used for the transformation. These should be clear on both the deed plan image, and the OS map, such as the intersections of walls, buildings, or streams, or corners of buildings etc. They should also be ideally located surrounding the subjects to be georeferenced.
6 Choose the first control point on the deed plan image and click on it. The number 1 will appear in pink. Pan and zoom to the corresponding point on the OS map and select it by clicking on it and the number 1 will appear in black.
7 Repeat this process until the required number of control points have been chosen, a minimum of four for Affine Transformation, and eight for Warp Transformation. Remember that control points should ideally be located surrounding the subjects, and not just in one part of the deed plan image as this will adversely affect the accuracy of the transformation.
8 Toggle the georeferencing tool panel visibility using the arrow at the top left-hand side of the screen.
9 The georeferencing tool panel allows points to be added, moved or deleted, depending on the result of the transformation. The transformation type is also chosen in this panel.
10 When Run Transformation is clicked the georeferenced image will now appear on top of the map in the right-hand side of the screen. The visibility of the image can be altered using the View Layers panel.
In the georeferencing tool panel, adjacent to each point is a RMS number and the direction of the error.
The RMS (Root Mean Square) is a measure of the difference between locations that are known and locations that have been interpolated or digitized, and provides a single, easily interpretable value that represents the overall error of the georeferencing process.
A lower RMSE value indicates a smaller average difference between predicted and actual co-ordinates, implying a more accurate georeferencing transformation. While a low RMSE is desirable, it doesn't guarantee perfect accuracy. A low RMS might be achieved by using poorly placed control points, so it is crucial to evaluate the placement and quality of control points as well.
11 At this point the accuracy of the georeferenced plan can be checked in relation to the OS Map.
If the georeferenced plan is deemed to be accurate enough to plot the boundaries onto the cadastral map, click Continue on the bottom right-hand corner of the screen.
If the georeferenced plan is not accurate enough to plot the boundaries on the cadastral map, then using the georeferencing tool panel
(i) additional control points can be added
(ii) existing control points deleted, or
(iii) existing control points moved (by clicking on the point on either the image or the map and dragging to desired position).
The transformation can then be run again, or the other transformation type selected and run. The georeferenced plan and RMS of each control point should be checked again to establish if the accuracy is now acceptable to plot the plan onto the cadastral map. If one point is shown to have a much higher RMS then delete that point and run the transformation to again to see if the overlay of the georeferenced plan is improved. This can be an iterative process.
Troubleshooting – if the transformations are not georeferencing the deed plan accurately enough to plot the subjects onto the cadastral map, then please consider the checks below.
i. Check that the deed plan is actually suitable for georeferencing per deed plan criteria and if not, follow the guidance in the manual on Plans References for Servitudes, Burdens and Non-Ownership Rights.
ii. If the plan is suitable, check control points are spread across the whole plan and residuals are within acceptable levels of the basemap. Having control points centred on one location and not surrounding the subjects will produce a poor transformation result.
iii. Check the transformation type selected is suitable for your plan.
iv. Refer up to plan creator business lead or referral point if any of the above does not produce acceptable results.
12 After selecting Continue, Plan Creator returns to the Edit screen. Polygons can be auto-filled and split along the boundaries of the georeferenced plan to map the subjects onto the cadastral map, or polyline features plotted follow the lines on the deed plan.
13 If the georeferencing process has not been finished, but the work done up to that point requires to be saved, or if the image with control points requires to be saved for future use, then by selecting Export the image is saved, and when this is uploaded again into the georeferencing tool, the image and control points previously chosen are present.
14 Once the plan has been georeferenced and plotted in Plan Creator, the image and control points can be exported per (13) above. If the image and control points are not required to be retained for future use, it can be deleted from the COF or DCU by clicking on the bin on the View Layers Panel next to the image.
Please note that the georeferenced plan will not be retained within the COF or DCU when the file has been saved.