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ZTVs (Zones of Theoretical Visibility) and ZVIs (Zones of Visual Influence/Impact) are well used terms amongst the planning fraternity. They are measures of the visual impact that a proposed development will have on the surrounding landscape. There are several such measures available and for years LSS has been at the forefront of this work.
Our approach has always been to determine what questions are being asked by experts in the field and to attempt to program commands which will answer those questions.
This is the list of commands currently available in LSS Elite
Visibility Point Feature: How many of a particular object type are visible from a grid of user-defined receptors?
Line of sight: What parts of a model are visible from a single position?
Line of sight to features: Are any parts of a development visible from a grid of receptors?
Visibility Profile: What would be the constraints of a development within which it could not be seen from a set of user-defined receptor positions?
Visibility Surface: How much of a receptor's vertical field of view is taken up by a development?
And now we have just added a new command called 'Horizontal ZTV' which measures how much of a receptor's horizontal field of view is taken up by a development. This is calculated from a user-defined grid of receptors and can measure either the largest contiguous feature visible, the total of all contiguous features or the maximum spread from the furthest left to the furthest right extent of the development. The information is stored as a horizontal angle in degrees. Both this command and 'Visibility Surface' are the only true tests of likely impact because the results reflect the effect that distance has on the apparent size of the object (a large object up-close has more visual impact than the same sized object further away [all things being equal]).
In the above example, we have four distinct objects, A, B, C and D. When viewed from position V at a precision of 2 degrees, object A takes up 9*2deg (18deg), B takes up 7*2deg (14deg) and C & D combine to produce a contiguous area of 14*2 (28deg).
Therefore, the maximum contiguous is 28deg, the total contiguous is 18+14+28=60deg and the total spread is 39*2=78deg. Any of these results can be produced for a user-defined grid of receptors and for a user-defined angular interval.
What is produced is a model where the elevation of every grid point represents the chosen Horizontal ZTV in degrees. It is then possible to contour this or display coloured bands in order to highlight potentially problematic areas of high 'impact'. And remember, in LSS if you add your own heighted links or surfaces into the model, these will act as barriers to the ZTV, thus allowing you to calculate a more realistic picture of the likely impact. And, furthermore, if you are using vector mapping information (such as MasterMap in the UK) LSS is capable of applying surfaces to different features, thus giving you a really quick way to apply heights to objects such as buildings and woodland.
This command is currently in test and available to use in the latest LSS Beta program.