b. When available, nonlinear graph paper should be used to determine LOS. Nonlinear
graph paper is designed specifically to consider both the curvature of the Earth and the effects of
refraction on radio transmissions. The graph papers shown in Figure 2-2 may be used,
depending on the elevations and distances of the proposed sites. The upper nonlinear graph
paper is used for elevations up to 5000 feet and distances up to 125 miles. The lower nonlinear
graph paper is used for elevations up to 500 feet and distances up to 50 miles. As indicated on
the two graphs, the vertical scale represents elevation and the horizontal scale represents distance
between the two points.
c. The following steps should be used to determine LOS using nonlinear graph paper:
(1) Step 1 - Determine from a contour map the scales used for the distances involved.
(2) Step 2 - Identify the locations of the two points on the map. Draw a straight line
on the contour map between the two proposed sites. Measure the length of this line (this
corresponds to the distance between the two sites).
(3) Step 3 - Determine the elevation at each site, as indicated by the contour lines.
Add the height of the antenna mast at each location to come up with the total elevation for each
site.
(4) Step 4 - Mark the location of one site on the nonlinear graph paper above the zero
(0) kilometer/mile elevation point corresponding to the total elevation of that site (site elevation
as determined by the map contour plus the antenna height). Mark the location of the other site
on the nonlinear graph paper on the elevation scale corresponding to the site's total elevation
above the distance point equal to the straight line distance between the two measured sites.
(5) Step 5 - Draw a straight line between these two points on the nonlinear graph
paper.
(6) Step 6 - Plot a profile of the terrain between the two sites. Follow the line drawn
on the contour map between the two sites (Step 2) and determine the heights of all significant
high and low points intersected by the line. Mark these points on the nonlinear graph paper
(elevation scale) corresponding to the obstacle's distance from one site. Join these points with a
line. If no obstructions are identified based on the above procedures, LOS exists between the
two sites. Any points plotted above the straight line drawn on the nonlinear graph paper (Step 5)
represent an obstruction to radio transmissions.
SS0121
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