viewshed - Areas visible from point on terrain elevation grid

Syntax

[vis,R] = viewshed(Z,R,lat1,lon1) viewshed(Z,R,lat1,lon1,observerAltitude) viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude) viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude, ... observerAltitudeOption) viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude, ... observerAltidueOption,targetAltitudeOption) viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude, ... observerAltitudeOption,targetAltitudeOption,actualRadius) viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude, ... observerAltitudeOption,targetAltitudeOption, ... actualRadius,effectiveRadius)

Description

[vis,R] = viewshed(Z,R,lat1,lon1) computes areas visible from a point on a digital elevation grid. Z is a regular data grid containing elevations in units of meters. The observer location is provided as scalar latitude and longitude in units of degrees. The visibility grid vis contains 1s at the surface locations visible from the observer location, and 0s where the line of sight is obscured by terrain. R is either a 1-by-3 vector containing elements:

[cells/degree northern_latitude_limit western_longitude_limit]

or a 3-by-2 referencing matrix that transforms raster row and column indices to/from geographic coordinates according to:

[lon lat] = [row col 1] * R

If R is a referencing matrix, it must define a (non-rotational, non-skewed) relationship in which each column of the data grid falls along a meridian and each row falls along a parallel. The value of R on output is identical to the value supplied as input.

viewshed(Z,R,lat1,lon1,observerAltitude) places the observer at the specified altitude in meters above the surface. This is equivalent to putting the observer on a tower. If omitted, the observer is assumed to be on the surface.

viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude) checks for visibility of target points a specified distance above the terrain. This is equivalent to putting the target points on towers that do not obstruct the view. if omitted, the target points are assumed to be on the surface.

viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude, ... observerAltitudeOption) controls whether the observer is at a relative or absolute altitude. If the observerAltitudeOption is 'AGL', then observerAltitude is in meters above ground level. If observerAltitudeOption is 'MSL', observerAltitude is interpreted as altitude above zero, or mean sea level. If omitted, 'AGL' is assumed.

viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude, ... observerAltidueOption,targetAltitudeOption) controls whether the target points are at a relative or absolute altitude. If the target altitude option is 'AGL', the targetAltitude is in meters above ground level. If targetAltitudeOption is 'MSL', targetAltitude is interpreted as altitude above zero, or mean sea level. If omitted, 'AGL' is assumed.

viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude, ... observerAltitudeOption,targetAltitudeOption,actualRadius) does the visibility calculation on a sphere with the specified radius. If omitted, the radius of the earth in meters is assumed. The altitudes, the elevations, and the radius should be in the same units. This calling form is most useful for computations on bodies other than the Earth.

viewshed(Z,R,lat1,lon1,observerAltitude,targetAltitude, ... observerAltitudeOption,targetAltitudeOption, ... actualRadius,effectiveRadius) assumes a larger radius for propagation of the line of sight. This can account for the curvature of the signal path due to refraction in the atmosphere. For example, radio propagation in the atmosphere is commonly treated as straight line propagation on a sphere with 4/3 the radius of the Earth. In that case the last two arguments would be R_e and 4/3*R_e, where R_e is the radius of the earth. Use Inf for flat Earth viewshed calculations. The altitudes, the elevations, and the radii should be in the same units.

Remarks

The observer should be located within the latitude-longitude limits of the elevation grid. If the observer is located outside the grid, there is insufficient information to calculate a viewshed. In this case viewshed issues a warning and sets all elements of vis to zero.

Example

Compute visibility for a point on the peaks map. Add the detailed information for the line of sight calculation between two points from los2.

Z = 500*peaks(100);
refvec = [ 1000 0 0];
[lat1,lon1,lat2,lon2]=deal(-0.027,0.05,-0.093,0.042);
     
[visgrid,visleg] = viewshed(Z,refvec,lat1,lon1,100);
[vis,visprofile,dist,zi,lattrk,lontrk] ...
   = los2(Z,refvec,lat1,lon1,lat2,lon2,100);

axesm('globe','geoid',almanac('earth','sphere','meters'))
meshm(visgrid,visleg,size(Z),Z); axis tight
camposm(-10,-10,1e6); camupm(0,0)
colormap(flipud(summer(2))); brighten(0.75);
shading interp; camlight
h = lcolorbar({'obscured','visible'});
set(h,'Position',[.875 .45 .02 .1])

plot3m(lattrk([1;end]),lontrk([1; end]), ...
   zi([1; end])+[100; 0],'r','linewidth',2)
plotm(lattrk(~visprofile),lontrk(~visprofile), ...
   zi(~visprofile),'r.','markersize',10)
plotm(lattrk(visprofile),lontrk(visprofile), ...
   zi(visprofile),'g.','markersize',10)

Compute the surface areas visible by radar from an aircraft 3000 meters above the Yellow Sea. Assume that radio wave propagation in the atmosphere can be modeled as straight lines on a 4/3 radius Earth. Display the visible areas as blue and the obscured areas as red. Drape the visibility colors on an elevation map, and use lighting to bring out the surface topography. The aircraft's radar can see out a certain radius on the surface of the ocean, but some ocean areas are shadowed by the island of Jeju-Do. Also some mountain valleys closer than the ocean horizon are obscured, while some mountain tops further away are visible.

load korea
map(map<0) = -1;
figure
worldmap(map,refvec)
da = daspect;
pba = pbaspect;
da(3) = 7.5*pba(3)/da(3);
daspect(da);
demcmap(map)
camlight(90,5);
camlight(0,5);
lighting phong
material([0.25 0.8 0])
lat = 34.0931; lon = 125.6578; 
altobs = 3000; alttarg = 0;
plotm(lat,lon,'wo')
Re = almanac('earth','radius','m');
[vmap,vmapl] = viewshed( ...
   map,refvec,lat,lon,altobs,alttarg, ...
   'MSL','AGL',Re,4/3*Re);
meshm(vmap,vmapl,size(map),map)
caxis auto; colormap([1 0 0; 0 0 1])
lighting phong; material metal
axis off

Over what area can the radar plane flying at an altitude of 3000 meters have line-of-sight to other aircraft flying at 5000 meters? Now the area is much larger. Some edges of the area are reduced by shadowing from Jeju-Do and the mountains on the Korean peninsula.

[vmap,vmapl] = viewshed(map,refvec,lat,lon,3000,5000, ...
               'MSL','MSL',Re,4/3*Re);
clmo surface
meshm(vmap,vmapl,size(map),map)
material metal
lighting phong

viewshed - Areas visible from point on terrain elevation grid

Syntax

Description

Remarks

Example

See Also

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