Drawing - Visible Area

The Visible Area command works with a surface and a drawing to find all parts of the surface that are visible from one or more locations in the drawing marked by points. This function in other GIS systems may be called a viewshed function or visibility zones function. This function is bi-directional in that it may also be used to find all locations from which the tops of one or more towers are visible from the surface.


The Drawing - Visible Area menu selection is enabled when the optional Surface Tools extension has been installed and the focus is on a drawing layer in a map window that also includes a surface layer. If you have not activated the Surface Tools extension with a valid Surface Tools serial number you will not be able to use the Drawing - Visible Area command.


The Visible Area command creates a new area object within the target drawing that shows the region of the surface that is visible from the designated points.




Launching the Visible Area command in the Drawing menu launches the Visible Area dialog. The dialog above uses a surface called Montara Mountain and a target drawing called Drawing (not a very original name, but descriptive).




Choose the surface for which visibility is to be analyzed.


Choose the drawing from which viewing point locations will be taken and within which the visible area will be created.


[All Objects in drawingname] or the selection or a saved selection, if one exists.


The data field to be used for heights of observing points. Choose [None] if no height field is to be used, in which case the observing points will be taken to lie upon the surface.

Relative Heights

If unchecked, interpret the height field to mean an absolute altitude. If checked, interpret the height field to mean a relative height above the surface at that location.

Compute area visible from all locations

If unchecked and multiple points are given the created area will be the region visible from any one of the locations. If checked, the area will represent only that part of the region that is simultaneously visible from all of the locations.


Using Heights


If [None] is specified in the Height field, Manifold will compute the regions of the surface visible from each location assuming that the observer's eye is located at ground level. That usually is not a realistic choice since a literally ground level view will easily be obstructed by slight rises that would not impede the view even from the height of an average person's eye.


It is much more frequently the case that we would like to know areas that are visible from a person's eye or from a tower or other structure. In such cases the eye of the observer is elevated to some height above the surface. The Height option allows us to specify the name of a column in the drawing's table that provides the heights of each viewing location.


At times the height of the viewing location is known in absolute terms (such as the height of an aircraft observer) and at times the height of the viewing location is known in relative terms (such as the height of a tower above ground level). The Relative heights option box allows us to choose which is the case for the height values in our drawing's table. Check this option box to use relative heights, for example, the height of a tower above ground level.




This example computes the area visible from five towers in the region of Montara Mountain, California. It uses a drawing of five points and a surface taken from part of the Montara Mountain USGS SDTS DEM data set.




Imported into the project the components appear in the project pane as seen above. The drawing (called Drawing) and the surface (called Montara Mountain) appear together in a map (called Map).




Opening the map we see that the drawing contains five points.




Opening the drawing's table, we see that each point has a Height value ranging from 100 to 10. The points have been thematically formatted in the drawing so that the color of each point varies from yellow (the 10 point) to purple (the two 100 points located on ridges).


The drawing represents five towers ranging in height from 10 feet to 100 feet that are located in various parts of the Montara Mountain region.


To find the visible area we click on the drawing tab in the map and launch Drawing - Visible Area.




In the Visible Area dialog we choose Montara Mountain as the Source, Drawing as the Target, [All Objects in Drawing] as the Scope and Height for the Height field. We check the Relative heights box because the height for each tower is given as the height above ground level at that position. Press OK.




The result is that a new area is added to the drawing. We have taken a moment to format the area in yellow color so that it is easy to see. Any location on the surface within that area is within sight of the top of one of the five towers. Because there are five towers and three of the towers are high towers, much of the surface is visible from at least one of the towers.


Suppose we would like to determine the visible area from only a subset of towers? That's easy to accomplish.




Let's suppose we are interested only in the area that is visible from either of the two tallest towers. We delete the new area created in the previous step, and then we select the two tallest towers (the purple dots).




We launch the Visible Areas dialog as before except this time the Scope is set to [Selection in Drawing]. Press OK.




The result is a new area that shows all parts of the surface that are visible from one of the two selected towers. Although it is a smaller area than is visible from all five towers, nonetheless it covers a large part of the surface because the two selected towers are tall towers that are located on high ridges.


So far we have looked at two cases where the visible area is visible from any one of the towers. Suppose we would like to find those regions that are visible from both towers at once?




We could delete the visible area created in the previous step, select the two tall towers and then re-run the Visible Areas dialog. This time we can check the Compute area visible from all locations box.




The result is a new area that shows all parts of the surface visible to both of the two selected towers at the same time. Because line of sight is bi-directional, if we stood at any of the locations marked by the yellow area we would be able to see the tops of both towers.


If we like, we can see which area is visible from all five towers simultaneously.




To do so we launch the Visible Area dialog using [All Objects in Drawing] for the Scope and we check the Compute area visible from all locations box.




The result shows that very few locations can be seen from all five towers at the same time. In general, it is only portions of the highest ridges that may be seen from all five towers at once. If we stood at any location within the yellow area we could see all five towers at once.




Only one area object is created by the Visible Area command. The area is a branched area object so that even those parts of the area that appear disconnected from other parts of the area all belong to the same area object. Clicking on any one part of the area using touch selection will select the entire area.


Strictly speaking, if we want to consider the visibility of a 100-foot tower we should add some value for the height of the individual who is viewing. One way to do this is to add some average amount, say, 5.5 feet, to the height of each tower.


Line of sight is bi-directional. Every spot that may be seen from the top of a tower is also a spot from which the top of a tower may be seen. The Visible Area command not only lets us find all locations that are visible from a given location, it also allows us to find all areas from which the given location is visible. This function is very useful in determining whether or not towers or other structures can be seen. For example, if we would like to determine the areas from which a cellular telephone antenna may be seen we can determine the visible area from the cellular antenna's location using the height of the antenna. Anyone standing within the visible area will be able to see the antenna.


This command is very useful for planning any line of sight task. For example, it can be invaluable when planning the location of monitoring instruments (such as seismic sensors on volcanoes) that connect to central stations via microwave or laser communications links, the location of observation posts for borders or secure installations or even the location of defensive positions in military installations.


Computing visible areas can be a highly computationally intensive task that can take a long time to accomplish with large surfaces or with many viewing points.


See Also


Surface Tools