Many Casual Cartographers use standard CAD packages, such as AutoCAD®, to create and
maintain maps using latitude and longitude coordinates. This is perfectly legitimate,
although your map will look somewhat strange, especially at higher latitudes. To be
successful using this technique, you need to be sure to adopt the following conventions:
enter the longitude as the X coordinate, and the latitude as the Y coordinate, and
use negative values for west longitude and south latitude.
Technically, the result is a map based on the Plate Carrée projection where the radius
of the sphere is assumed to be 57.29577951 (assuming you enter the latitude and longitude
in degrees). Casual Cartographers who use this technique can then work in terms of
latitude and longitude, and then use a product such as our own Tralaine to project the
result whenever they require a more accurate representation of the geography. Essentially,
when applied to an AutoCAD drawing file, or the graphic file produced by other CAD
packages, Tralaine’s Latitude and Longitude family of coordinate systems assume the
above conventions.
When using this technique, however, some Casual Cartographers get confused about the
use of degrees, minutes, and seconds in this environment. In this article, I attempt to
eliminate this confusion.
Take a moment and consider the following:
Perhaps you will have recognized that all of these images are different representations
of the same thing, i.e. the integer value 13. The second and fourth representations are
hexadecimal and binary; representations used by us computer nerds. However, each variation
uses a different technique, but the message is the same; namely the integer value 13.
The following are three different representations of the same thing, namely an angle of
32 and one eighth degrees:
32-1/8 32.125 32°07.5’ 32°07’30"
Just as in the above example, these are four different means of expressing the same
thing, 32 and one eight degrees. The meaning of the expression is the same; the only
difference is its representation.
Internally, a computing system deals with numbers in a consistent fashion. For obvious
efficiency reasons, there cannot be separate circuitry to deal with all of the possible
forms of representation of numeric values. That computer systems are capable of
understanding different forms of numeric representation is a function of the software
which operates on the computer.
AutoCAD is a very capable and popular software product. Wherever an angle is expected
or acceptable in the syntax of a command, you can enter angles in either decimal form
(32.125) or in degrees, minutes, and seconds form (32d7’30"). To use degree,
minute, and second form, AutoCAD requires you to provide the lower case ‘d’ as a
clue that the degree, minute, and seconds for is being provided. Thus, to enter degrees,
minutes, and seconds form, the angle must be entered as:
32d7’30"
Given the ‘d’ clue, AutoCAD knows to interpret this representation as
degrees, minutes, and seconds, and converts it to the standard form used in computer
systems, namely 32.125. Thus, for all practical purposes, AutoCAD does indeed support
degrees, minutes, and seconds.
However, there are two catches to the technique implemented by AutoCAD. First is the
phrase "wherever an angle is expected or acceptable". Second is that AutoCAD
assumes that anything entered which contains the ‘d’ clue is indeed an angle;
not a distance, not a scale factor, but an angle. Except for the very specific polar
coordinate form, AutoCAD does not expect angles when it expects a cartesian coordinate;
nor does it consider an angle an acceptable at this point in the syntax. Therefore, while
AutoCAD does indeed accept degree, minute, and second form, users cannot use this form to
specific a coordinate; i.e. the location at which to place a block. Thus, Casual
Cartographers may be lead to say the AutoCAD does not support degrees, minutes, and
seconds; and to a certain degree, but only to a certain degree, they are correct.
When you are using the technique of maintaining maps using latitude and longitude
coordinates, you are essentially entering angles as coordinates. Essentially, AutoCAD is
considering them to be distances. When you enter 32.125, AutoCAD has no reason to suspect
you really mean 32.125 degrees, it simply assumes you mean 32.125 drawing units. However,
if you enter 32d7’30" at a point where AutoCAD is expecting an X coordinate, it
will reject the input as a syntax error; you provided an angle when a distance was
required.
In review, there are 3 important points:
AutoCAD does indeed support the degree, minute, and second format for all angles.
For normal X and Y type coordinates, AutoCAD expects/requires two distances, and will
reject angles (except in the case of the polar coordinate input format).
You can essentially input numeric values known to you to be angles, providing you
don’t put them in a form which AutoCAD knows is an angle.
So, the trick is to convert your degree, minute, and seconds numbers to standard
decimal form before keying them into AutoCAD. How is that done? Most electronic
calculators have the ability to do this. I find it often difficult to remember and/or find
the right buttons on my calculator which has (seemingly) hundreds of them. And then the
precision available is often insufficient. The following table shows the procedure for
converting degree, minute, and second format to decimal format. In the example we’ll
assume the value to be converted is 12° 34’ 56.789":
Converting from decimal form to degree, minute, and second form is, quite naturally,
the inverse of the above:
