Viewshed, view corridor, and what actually drives the cost of a shoot

In geographic information systems, a viewshed is the set of all locations that are visible from a given observation point. It is computed by tracing a line of sight from that point to every cell of an elevation raster and asking whether anything in between rises above the line. The output is binary: each cell is visible or it is not. Everything else about the analysis is a question of what you fed into it.

That last point is where most disagreements start. A viewshed computed on a bare earth terrain model sees through buildings and trees, because they are not in the model. One computed on a digital surface model, which includes everything the sensor saw, gives an answer that looks nothing like it in a city.¹ Neither is wrong; they answer different questions. Anyone quoting a viewshed should say which surface it was computed on.

Viewshed

Everything visible from one point, computed on an elevation model

View corridor

A sightline protected by zoning, not by geometry

Observer offset

Eye height added above the model surface

Horizon at 200 ft

About seventeen miles over a smooth earth

The four parameters that change the answer

A viewshed is only as meaningful as its parameters, and there are four that matter more than the rest.

• Observer offset. The height added above the model surface at the observation point. For a view study this is the eye height above the finished floor, not the floor elevation itself.
• Target offset. The height added at every other cell. Set it to zero and you are asking what ground is visible. Set it to twelve metres and you are asking what rooftops are visible, which is a different map.
• Radius. An unbounded viewshed will happily report visibility at fifty miles, where nothing is legible to a camera or an eye.
• Curvature and refraction. Over any distance worth analysing, the earth curves away and the atmosphere bends light back down. Analyses that correct for both typically apply a refraction coefficient of around 0.13.²

The curvature term is not academic at the heights this site is about. Over a smooth earth, the geometric distance to the horizon in statute miles is roughly 1.22 times the square root of the eye height in feet.³

Adding atmospheric refraction extends each of those figures by something like eight per cent. In a real landscape none of it survives contact with the first hill, which is exactly why the computed viewshed is worth having.

A viewshed tells you where you could be seen from. It says nothing at all about whether anything there is worth photographing.

The limit of the analysis

View corridors are legal objects, not geometric ones

A view corridor, sometimes called a view cone, is a sightline that a municipality has decided to protect, usually by restricting building heights within a mapped wedge of land. Vancouver in British Columbia is the best known North American example, and a number of other cities operate comparable protections under different names.⁴

The distinction matters when a client uses the two words interchangeably. A viewshed is what physics allows today. A view corridor is what an ordinance protects tomorrow. A development can sit entirely inside a beautiful viewshed and lose the whole thing to a neighbour building within their own entitlement, which is the point made at more length in the page on preconstruction view studies.

What actually drives the effort in an aerial job

Once the viewshed is drawn, the shoot plan follows from it: how many observation points are needed to cover the question, and what elevation each one has to reach. That is the honest basis for scoping the work, and it explains why height alone is a poor predictor of effort. The following are the variables that multiply it. None of them are prices, and this site quotes none.