FoxPlot : A DF Plotting Program for PDAs



FoxPlot is a PalmOS "utility" plotting program intended for use by transmitter hunters. Basically, FoxPlot is a PDA "substitute" for a map, a protractor and a pencil… it allows the user to enter the latitude and longitude of nine DF locations, along with the DF bearings taken at those locations, and then plots the DF lines on a ( featureless ) "map", to show where they intersect. This can be particularly useful for pedestrian hunts, where carrying a PDA is preferable to carrying the other items. ( and trying to find a flat hard surface to use them )

The program itself has a DISPLAY page and an INPUT page. Switching to the INPUT page is achieved by pressing either the UP or DOWN buttons on the PDA case… these are the two buttons located one above the other, at the bottom / center of the PDA. The DF data is stored in a database file, so the program can be closed and re-opened later, without loss of the plot data. When first installed and started, FoxPlot will automatically create a database file and load it with "dummy data" for 7 DF bearing lines.


The Data Inputs

Data must be manually entered for each of the nine locations, using the touch-sensitive screen on the INPUT page. Data is entered simply by pressing directly on the digits to be changed… the digits will increment each time they are pressed. Latitude and Longitude data must be expressed in degrees, minutes and fractions of a minute. ( NOT in degrees / minutes / seconds ) DF bearing data must be entered in true degrees. ( NOT in magnetic or relative degrees ) Individual locations and bearing lines can also be "switched off", so they do not clutter the display. ( for example, if they are empty and contain no valid DF or lat / long data )


The Display Screen

The latitude and longitude for the center of the screen is displayed ( numerically ) at the bottom center of the DISPLAY page. The DISPLAY screen can be scrolled in any direction ( N, E, S or W ) by touching the screen at any point other than the center…. the touch point will become the ( new ) "screen center" and the display will be re-plotted for this new location. The lat / long readouts on the bottom will also update to reflect the change.

Using this feature, the user can also touch the screen at the intersection point for the various DF bearing lines, and that point will immediately be moved to the center of the screen, with a corresponding lat / long display for the point, at the bottom of the screen. Since the original lat / long data will very probably come from a handheld GPS unit, the resulting lat / long data can then be entered into the GPS as a waypoint, and the ( sophisticated ) navigation features of the GPS can then be used to calculate the bearing and range to the intersection point.

The center of the screen can also be "manually" defined on the INPUT page, in location "0". This is useful if the location of the screen center has moved a great distance ( more than 100 miles ) since it was last used. ( saves a lot of screen tapping )

The displayed range can be increased or decreased by pressing the IN or OUT buttons on the DISPLAY screen. Maximum range is 100 miles, minimum range is 100 feet.


Caveats and Bugs

The FoxPlot program has a few bugs…

BUG 1 : The program will "crash" if the DF bearing is an exact "cardinal" value : 000, 090, 180 or 270 degrees. If this is the DF bearing value you get from your equipment, add or subtract 1 or 2 degrees before entering it.

BUG 2 : It is sometimes necessary to tap the display screen twice to define a new "screen center". This is an artifact of the operating system, and may not be "repairable", but hardly a big problem.

BUG 3 : Each bearing line on the display is identified with a small circular "bug" that encloses a number, corresponding to the data for that bearing line. ( 1 to 9 )

If the lat / long location for that particular DF point "fits" onto the display screen, ( = display range is large enough ) the bug will be "painted" at that particular lat / long, and a radial line will be drawn from the bug to the edge of the screen, on the DF bearing. ( as one would expect )

If the lat / long for that DF point lies "off screen", but the radial line from that point "crosses" the visible portion of the screen, then the visible portion of the line is "painted" on the screen, ( the visible line "segment" ) and the bug for that line is "painted" at one end of the line. ( to identify which DF data corresponds to that line segment ) Due to a software problem, the identification bug is not always painted at the "source" end of the DF line segment, and may instead be painted and the "destination" end of the line segment… this can be a little confusing, and will eventually be fixed.

BUG 4 : In a similar manner, if the source lies "off screen" and the DF bearing line also lies "off screen", another software problem can cause the RECIPRICAL bearing line to be "painted" ON the screen, IF that RECIPRICAL bearing line crosses the visible portion of the screen.


This is an artifact of the math, since a "line" is defined ( in algebra ) with no starting or ending points, but a DF bearing line has a specific starting point but no ending point. This will eventually be fixed, but be aware of it…