DTM Visibility Analysis

The DTM Visibility option allows you to carry visibility analysis for one of two possible scenarios.  The first is showing what can be seen from a given point and the second is showing where a point is visible from.  It is not only a useful tool in supporting planning applications but can also be used to help in the design bunds for landscaping and screening work.

When you select this option, the first thing you must do is to indicate the point of interest.  This may be the top of a building which you are planning to build.  It could also be the front door of somebody’s house where they have objected to a planning proposal.  After selecting the point, a property sheet allowing you to set the parameters of the analysis is displayed.

On the Settings page of this sheet, there are two fields called Origin Level and Offset Height and it is important to understand what values you need to enter in these two fields.  The Origin Level field will have a default value which is the height of the point that you have indicated.  If you are analysing what you can see from the indicated point, you would normally increase the value in the Origin Level by your height and set the value in the Height Offset field to be 0.0.  If you are analysing where the indicated point can be seen from, you may leave the value in the Origin Level as it is and set the value in the Height Offset to be your height.  In the situation where you would like to analyse the impact of a new building, you should add the height of the new building to Origin Level field.

The visibility analysis uses radial sections as its basis.  Each section is analysed based upon a start height and an observer height to see which parts of the DTM are obscured by its topography.  The elements of the sections which are visible or hidden can then be plotted in different colours as radial hatches.  You can use the Plot Visible and Plot Hidden check boxes to control which elements to create.  Colours and layers are controlled by the settings in the Plot Attributes group.  The Show Sections check box allows you to view the sections that are created to carry out the analysis.  If they are shown, you have the option to save the sections for later plotting.

The Bearings group defines the directions at which the cut lines radiate out from the point of interest.  You can constrain the sector to which cut lines are created using the Start and Finish fields.  The cut lines will start at the start bearing and be generated around the point of interest in a clockwise direction until the finish bearing is reached.  If you were only interested in the northeast quadrant, the start bearing would be 0° and the finish bearing would be 90°.  The Interval field is the angular interval between cut lines.  Using the example above, an interval of 10° would generate 10 cut lines.  The Full Circle check button allows you to specify that you wish to take sections through 360°.  The Distances group defines the length of the cut lines.  The values in the Start and Finish fields define the distances from the point of interest of the start and finish of the cut lines.

It is often the case that you are carrying out a visibility analysis of a design model and would like to include a secondary model containing additional data representing the area surrounding the design.  This data could come from other sources, such as LIDAR.

The Models page allows you to select this secondary model.  You should ensure that the design model remains at the top of the Selected list and move the secondary model across.  Any further models will be ignored.  When the visibility analysis is carried out, the design model has priority and the secondary model is only used where there is no vertical profile from the design.  Effectively, the design model is cut into the secondary model.  After you have set all required settings, the sections will be calculated, and the visibility analysis performed.  The required hatches representing visible and hidden detail will be created.

In the example above, it was required that a tall building be built inside a quarry.  The quarry is represented by the grey area in the centre of the image.  The yellow shaded areas show where the top of the building is hidden from view whilst the red shaded areas shows where the top of the building is visible.