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Drawing a Map from Instrument Data
(Very basic instructions suitable for an introductory session to mapping.)
(Click here for a PDF version)
Each shot taken with a mapping instrument represents a single point on the ground, and in constructing a map we are simply translating these points to paper.
Begin by determining the distances and elevations of each shot you are going to plot:
Determine the distance of each shot by subtracting the bottom intercept from the top intercept and multiplying by 100. This gives you the distance in meters.
(T.I. – B.I.) x 100 = distance in meters
If there was an obstruction and the top intercept is missing, subtract the bottom intercept from the middle intercept, double this, and then multiply it by 100. If the bottom intercept is missing, subtract the middle intercept from the top, double it, and multiply it by 100. These methods are not as accurate as using the top and bottom intercepts, but they will work in a pinch.
(T.I. – M.I.) x 2 x 100 = distance in meters
(M.I. – B.I.) x 2 x 100 = distance in meters
Determine the elevation of each shot by subtracting the middle intercept from 100. This
will effectively normalize the elevations and allow us to draw accurate
topographic lines on the map with no further mathematics. Using "100" is
arbitrary and we can translate the topographic lines to true elevations later. We
are simply using 100 meters as an arbitrary baseline elevation.
100 – M.I. = normalized elevation
To determine the elevation of shots taken from instrument station #2, first determine the elevation at station #2 (this will have been shot in from station #1), add to this the instrument height, and simply use this number, instead of 100, as the number from which middle intercept readings are subtracted (this number is the new arbitrary baseline elevation). Follow the same procedure for determining elevations of shots taken from instrument station #3 – first determine the elevation of station #3 (by subtracting the middle intercept from the number determined above for station #2), add to this the instrument height, and use this as the number from which middle intercept readings are subtracted.
100 (Arbitrary baseline elevation)
– (Station #2 elevation + Station #2 instrument height)
= (New arbitrary baseline elevation)
Mark a point on the paper to represent the instrument station.
Try to locate the instrument station on the map so that all of the shots will fit on the rest of the paper – if the shots were mostly to the north, for instance, place the instrument station near the southern part of the map. If the map covers a large area, you may have to use a large sheet of paper or tape two or more pieces together. Use the instrument height as the middle intercept to determine the elevation, and write the elevation next to the mark.
100 – (instrument height) = normalized elevation at instrument station
Center the protractor over the instrument station and align 0 degrees with the top of the page.
This orients 0 degrees with north – the protractor now represents the base plate of the instrument as we had it aligned in the field. Aligning the protractor with the top of the page will be much easier if you draw the map on graph paper.
Find the azimuth on the protractor that corresponds to the azimuth of the shot you are going to plot, and make a light pencil mark next to it on the paper.
(Remember that we read azimuths clockwise – some protractors have scales both ways.) A line drawn from the instrument station through this mark would represent the line along which the instrument was aimed in the field. We don't need to draw this line on our map, we just need to find one point along it.
Place a ruler along a line from the instrument station to the azimuth mark you made, with the 0 mark of the ruler at the instrument station, and draw a mark along this line corresponding to the distance of the shot.
We will use the scale 1 centimeter = 5 meters. This means that each centimeter on the ruler corresponds to 5 meters on the ground. If the shot distance is 50 meters, it is 10 centimeters along the ruler, etc. At this scale, two millimeters on the ruler = 1 meter on the ground.
The location of this mark on the map corresponds to the location of the stadia rod on the ground when the shot was taken. You should now erase the earlier azimuth mark you made so you don't confuse it with a shot location later – if the shots are close together it can become confusing.
Write the normalized elevation of the shot next to the mark, and label it if necessary.
If the shot was simply a "topography" shot, it doesn't need a label, otherwise lightly write "NE corner of Old Main", "center of sidewalk", or whatever description is given in the "shot description" column of the instrument mapping log.
Once you have plotted all the points, simply "connect the dots" to complete the objects on the ground.
Draw the known edges of buildings, sidewalks, or anything else that was mapped. You can erase the notes you wrote by the shot locations explaining what they were, but leave the marks that locate each shot on the map, and leave the elevations by each of these. We will use these to draw topographic contours on the maps in class.
Don't forget to complete you map by including the following 5 elements:
Direction (north arrow)
Scale (graphic and not just numeric)
Key (at the edge of the map or written next to the mapped elements)
Name, date, and project
Information putting the map in a wider spatial context
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