RP Elevations by TheoLt & QGIS

Required tools

• AutoCAD /BricsCAD
• TheoLt http://www.theolt.com/downloads/TheoLtR10.exe
• Image processing tools: Light room/Photoshop/MS Paintshop
• QGIS https://www.qgis.org/en/site/forusers/download.html#

QGIS is a freeware, open source, GIS compiler for map based database work. It includes a useful geo-referencing tool which will re-project images onto map co-ordinates. QGIS (like most GIS systems) does not work on vertical planes so care is need to get the image projection to work in the required plane.

Both CAD and QGIS applications will be run in a common Windows session.

WORKFLOW

Site capture:
Use TheoLt default orientation align with plane option for the first instrument set up for the principal facade. Trace wire frame in TheoLt of unambiguous target points/ edges/features. Edge traces of windows and building lines etc. Beware oblique shots. You may use reference positions (RPOS_ID) to mark points but ‘natural points’ (door window gutter line etc) are easier to interpret in CAD.
Acquire elevation photography (try to take shots ‘square on’ with minimum tilting of the camera). Even exposures between shots is best. Check the shots cover and resolve the required detail. The North side of the building is always a problem, it may need a visit in good light just to get the best shots.

CAD prep:
Set up UCS for the required facade planes. Save the UCSs with logical names

Image prep:
Download images to project folder. Apply rotation (if in portrait format rotate to upright) Apply raw conversion/down sampling (for large project or overlapping cover caused by limited stand off). QGIS will not handle RAW files.

Rectify in QGIS:
With the CAD dwg open and in the required UCS open QGIS, select the Georeferencer tool, select the co-ord system, select ‘open raster’ and mark the control points on the image using the ‘add points’ command. The X Y values can be lifted from the CAD dwg by id and command line copy paste. Take care to get the axes correct! Rectify the image with ‘start geo-referencing’ option

Import rectified image into CAD:
The image will need re-scaling to match the control points on the wire frame, this will be a fixed scale factor for the job. Move and rotate the scaled rectified image to match the control points.

Digitise:
Trace features off rectified image onto the facade plane.

PROCEDURE

Note that the ‘GEOREFENCER’ plug-in is required, it’s supplied as a default. The Georeferencer plugin needs to be ‘acivated’ prior to 1st use. From the ‘Plugins’ menu select ‘Manage & Install plugins’, select Georeferecer from the list and check the box to activate.

Activate-GEO-1

1.Open QGIS

DT-Icon
Of the 5 desktop short cuts it installs use ‘QGIS Desktop’. QGIS opens with the Project panel:

Project-panel-4

2.Open Georeferencer ( under the ‘Raster’ menu)

Open-Geo
Select the image and the Coordinate Reference System Selector panel opens:

Select-CS

Select ‘User defined co-ordinate system’.

qgis-2

You are returned to the Georeferencer panel with the image parked in it go to ‘Settings’ menu and select Transformation settings.
• Select Projective as transformation type
• Set file name for output image in Output Raster

If rectification is attempted without the Transform settings they will be prompted for prior to writing the file.

3. In CAD set up a UCS projection plane on the TheoLt wire frame.

Set the UCS icon to show its origin:

Command: UCSICON
Enter an option [ON/OFF/All/Noorigin/ORigin/Properties] <ON>: OR

Set the Visual style to 2Dwireframe
Setvar
UCSVP=0
DUCS=0
UCSDETECT=0
The axes of the elevation UCS must be logical (X across, Y up) and have positive values across the facade UCS.

Method:
Identity 2 points on the facade. Snap a polyline in WCS through the facade points. Extend the line beyond the facade plane to ensure +ve co-ords. UCS ‘e’ or ‘o’ to create a UCS snapped to the polyline.
Check the origin of the UCS is to the bottom left of the facade as viewed in elevation.
View the UCS:

Command: PLAN
Enter an option [Current ucs/Ucs/World] <Current>:
Regenerating model.

Move the origin of the UCS by reverting to WCS and moving the polyline in Z:

Command: m
MOVE
Select objects: 1 found
Select objects:
Specify base point or [Displacement] <Displacement>: Specify second point or
<use first point as displacement>: @0,0,-5

UCS ‘e’ or ‘o’ to create a UCS snapped to the polyline. Rotate the UCS axes so that Z is pointing OUT of the facade.

Command: ucs
Current ucs name: W Elev
Specify origin of UCS or [Face/NAmed/OBject/Previous/View/World/X/Y/Z/ZAxis]
<World>: x
Specify rotation angle about X axis <90>:

UCS-Set-up-_01

When the origin of the UCS is to the bottom left of the facade as viewed in elevation save the UCS.

4. Mark control points on the image in QGIS
Use the add point command to pick points on the image and input the co-ordinates in the ‘Enter map coordinates’ pop up panel.

qgis-6

The points are marked on the image as a red dot. In ‘add point’ mode any click in the image area will prompt for a coordinate. Pick the hand icon to switch to a pan and zoom mode.

qgis-3

The co-ordinates are extracted from CAD by ID of endpoints on the wireframe. The X and Y are copied and pasted individually.

qgis-5

Points in error can be removed by right click sub-menu options in the GCP table below the image. A minimum of 4 points are required.

The rectification will be local to the facade plane, the co-ordinates must be taken in the façade UCS do not revert to WCS .

The image is re-projected by using the ‘Start Georeferecing’ (green arrow) button. If the settings are not determined, on selection of the ‘Start Georeferencing’ button a prompt for the transformation type appears.

qgis-7

At ‘ok’ the ‘Transformation settings’ panel will open to allow the output file to be set. The transformation type is ‘Projective’.

qgis-8

The re-projected image file type will be Tif.

The image is now re-projected to match the chosen points. It may now be considered as rectified to the plane described by the points.

If the points are miss-matched QGIS will be unable to project the image and hang.

5. Insert rectified image into CAD

In CAD insert the re-projected image, it will be placed on the current UCS by default.
Choose an arbitrary insertion point and place the image at any size.
• Move the image insertion point by common point match.
• Scale the image by reference to common points

UCS-Set-up-_02

QGIS Georeferener works in pixels so the scale values are arbitrary, this is not a problem as the scale  can be dealt with as CAD operation- the perspective and rotation in the image will be corrected for the plane determined by the control points.

RP-IN-CAD

Note on scale & precision.

Rectified photographs are only precise where the objects in the photo are coincident with the plane of rectification they will only be reliable on a plane by plane basis: you cannot, for example trace the ridge off a roof line from an RP of the front of a building.

Elevations traced off RP are as precise as the match of the plane of rectification to the façade. As you move into or out from the plane the scale of the lines will deviate from that determined by the plane of rectification. The scale shift may be within the scale tolerance for the survey or it may not.

Note on scale tolerance.
At 1:50 scale the smallest plot value (roughly 1mm ON THE PLOT ) is 2.5cm in real world. If objects vary in size more than 2.5cm outside the plane of rectification they will be out of tolerance. Another way of thinking about this is to consider the planes of the façade: if an object is more than 2.5cm in front or behind the plane of rectification it will be out of tolerance for a 1:50 elevation.

At 1:100 scale the smallest plot value (roughly 1mm ON THE PLOT ) is 5cm in real world. If objects vary in size more than 5cm outside the plane of rectification they will be out of tolerance. Another way of thinking about this is to consider the planes of the façade: if an object is more than 5cm in front or behind the plane of rectification it will be out of tolerance for a 1:50 elevation.

At 1:20 scale the smallest plot value (roughly 1mm ON THE PLOT ) is 10 mm in real world. If objects vary in size more than 10mm outside the plane of rectification they will be out of tolerance.
If the facade has multiple planes multiple rectifications are necessary.

B

 

About billboyheritagesurvey

Heritage worker
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One Response to RP Elevations by TheoLt & QGIS

  1. Reblogged this on Billboyheritagesurvey's Blog and commented:

    Updated for QGIS 3.0.2…

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