A scale that will actually measure Latitude AND Longitude on TOPO maps.
To create a line of any length by describing its end points as the intersection of the line (scale) and a "V" which moves up or down in relation to the line. The line is divided into increments by projecting a curved scale onto the line using a pivoted swing arm.
To Measure Longitude of a point on a topo map
1. place the VARISCALE on the map so that one of the horizontal lines (usually the middle one works best) aligns with the lower edge of the map or with two tick marks* that lie below and on either side of a point of interest. Hold the VARISCALE firmly on the map by pressing on the outer edges of the scale with your middle fingers. Use your thumbs to gently slide the template (the template contains the "V" and the swing arm) up or down until both sides of the "V" cross the horizontal line exactly at the tick marks. We refer to this as "Setting The Working Length".
It is important that the horizontal line stays aligned with the map and the tick marks.
2. Next, gently slide the vertical bar left or right by applying slight pressure on the lower rivets until one of the vertical lines coincide with your point of interest. ( This projects the point of interest onto the horizontal line.)
3. Move the swing arm until its hairline crosses the intersection of the vertical and horizontal lines. (This projects that intersection onto the curved scale - or if you prefer, think of it as the scale, at that precise point, is being projected onto the line)
4. Since the scale reads left to right or right to left you can add or subtract your scale reading to/from the known longitude represented by the printed tick mark on the map.
Note that the lines of longitude converge (get closer together) as you go north or south of the equator, and you must therefore re set (calibrate) the VARISCALE anytime you measure points that vary in latitude.
To measure latitude you only need to set the VARISCALE one time for the map you are working with. The method of setting the VARISCALE is the same as for longitude. (except - turn the map 90 deg. )
* Most quad sheets have internal tick marks indicating known latitude and longitude. On a 7-1/2 minute quad sheet the tick marks are 2-1/2 minutes apart or 150 seconds which curiously is exactly the number of graduations on the VARISCALE.
To Measure Distance
In this example we will use the distance scale printed in the legend of a map however the same principle could just as easily be used to set/calibrate the VARISCALE to a scaled drawing using a known distance such as a sheet of building material of known dimension.
On the same map as above (USGS 7-1/2 minute quad) locate the legend.
This time we will define the working length a little differently. For example let's use the 10,000 foot distance scale.
1. Set the hairline on the swing arm to read 100, reading from left to right.
2. Now use the left side of the "V" and the swing arm (instead of the right side of the "V" to set the working length.
3. Using the same method as described above set the working length to 10,000 feet.
The range the scale will measure is now 15,000 feet and the tick marks on the curved scale are equal to 100 ft.
Top of page