Advanced
Intervisibility
Analysis
Visibility impacts a soldier’s ability to operate by
constraining his or her field of view, viewing range,
and engagement opportunities. It is a key component
of many traditional tactical decision aids, including
line-of-sight profiles and viewsheds.
Traditional tactical decision aids, while valuable, only
provide a limited understanding of the overall
visibility characteristics of the terrain. Both line-ofsight profiles and viewsheds are based on a single
observer location, so they represent a small fraction of
the overall visibility information.
To address this deficiency, University of Edinburgh
and U.S. Army researchers combined their interests in
visibility research and distributed computing to
explore the next generation of visibility products.
Local Viewshed
Local Core Visibility
(8-Connected)
The local viewshed is the foundation of the
Complete Intervisibility Database (CID).
The CID stores a viewshed for an observer
at every post in the Digital Elevation Model
(DEM). Workstation ArcInfo’s VISIBILITY
command is used to compute the viewsheds
for sixteen (16) observer locations in a
single operation. The resulting viewsheds
are converted to a Band Interleaved By
Line (BIL) format file and zipped for
storage.
Local core visibility describes the area
which is both visible and contiguous to the
observer’s location. Core visibility can be
computed using a 4-neighbor or an 8neighbor definition of contiguity. Local core
visibility is militarily significant, because a
soldier can maintain continuous visual
contact when approaching any target
located within the core area.
Local fragmentation is the number of
connected regions in a viewshed. Local
fragmentation can be computed using a 4neighbor or an 8-neighbor definition of
contiguity.
This measure is computed
outside of Workstation ArcInfo, but it is
equivalent to the application of the
REGIONGROUP command to the visible
areas and the SETNULL command to the
non-visible areas.
Cumulative Visibility
Cumulative Core Area Visibility
Cumulative Fragmentation
(8-Connected)
Cumulative visibility records, for each point
in the DEM, the total number of visible
pixels in the viewshed, the visible area, or
the percent visible area. When counting the
number of pixels in viewsheds, the value
can range from 1 to the total number of
points in the DEM, with higher values
indicating locations with greater visibility.
Cumulative visibility is useful for rapidly
identifying areas of low and high visibility in
a DEM and can be used as a cost surface for
selecting the least and most visible routes.
Cumulative core area visibility records, for
each point in the DEM, the total number of
local core visible locations, the local core
visible area, or the percent local core visible
area. The value for each point in the
cumulative core area visibility map can
range from 1 to the total number of points
in the DEM, with higher values indicating
larger local core visibility areas.
Cumulative fragmentation records, for each
point in the DEM, the local fragmentation
for an observer at that point. The lower
limit of this measure equals one, with
higher values indicating locations that have
greater viewshed fragmentation. It provides
an indication of the degree of connectedness
of the viewsheds. Fragmentation is largely
controlled by the nature of the intervening
features.
Local Fragmentation
(8-Connected)
Using ArcGIS, the research team developed a series of
applications to create, store, and analyze viewsheds for
every post in a Digital Elevation Model (DEM). This
new visibility product is called the Complete
Intervisibility Database (CID).
To create the CID, researchers turned to the power of
distributed computing. This approach solved the
problem, which was too complex and resource
intensive to be practically addressed by a single
workstation. Using a distributed computing solution,
multiple computers were accessed and tasked as their
resources permitted.
The CID forms the foundation for a variety of products
that describe the overall visibility characteristics of the
terrain. These products take local information from a
single observer location and summarize it in a variety
of cumulative products, such as cumulative visibility,
cumulative core area, and cumulative fragmentation.
The insight derived from these products provides a
deeper understanding of the nature of visibility.
Nine Sample Viewsheds from
the Complete Intervisibility Database
of Southwest Harbor, ME
Source Data
All images on this poster were
derived from a Complete
Intervisibility Database (CID) of a
US Geological Survey Digital
Elevation Model (DEM) covering
Southwest Harbor, Maine. The
DEM contained 466 rows and 336
columns, resulting in a CID with
156,576 viewsheds.
Slope of Cumulative Visibility
Ratio of Cumulative Visibility to
Cumulative Core Area Visibility
The slope of cumulative visibility measures,
for each point in the DEM, the slope of the
cumulative visibility values associated with
that point. The value can range from 0 to 90
if the slope is calculated in degrees, or 0 to
infinity if the slope is measured as a
percent. Higher values equate to steeper
slopes. Slope can be used as a guide for
changes in visibility characteristics, but it
does not have the conventional meaning of
slope, because the X, Y, Z units are not
equivalent. It can be used to identify
potential ambush sites.
The ratio of cumulative visibility to
cumulative core area visibility can range
from the total number of points in the DEM
to 1. Higher values are associated with
locations that have small local core visibility
relative to their viewshed. When the ratio
equals one, the cumulative visibility equals
the cumulative core area visibility and the
observer’s entire viewshed is limited to the
core area. This ratio gives the soldier an
idea of the relationship between the local
core area visibility and the viewshed.
D.R. Caldwell
U.S. Army Engineer Research and Development Center
Topographic Engineering Center
7701 Telegraph Road, Alexandria, VA, 22315-3864, USA
Tel +1 703-428-3594
Fax +1 703-428-3732
Email: Douglas.R.Caldwell@erdc.usace.army.mil
M.J. Mineter, S. Dowers, and B.M. Gittings
Geography, School of GeoSciences
University of Edinburgh,
Drummond Street, Edinburgh EH8 9XP, Scotland
Tel +44 131 650 2662
Fax +44 131 650 2524
Email: {mjm, sd, bruce}@geo.ed.ac.uk