APRS Area Objects

APRS allows a handful of objects that are not just point data, but specify an area.

Creating area objects

Select the "Area object" toggle button in the Object/Item create dialog. The dialog will change so that you can enter the parameters of area objects and grey out any that you may not modify.

You will be able to select the type of object, whether or not it is filled, and what color it should be. The base colors are selectable with a toggle button, and you can also select whether to make them "bright" colored or not.

Area object sizes are not intuitive

The "Offset Up" and "Offset Left" parameters determine the object's size relative to the point selected for the object's symbol itself. These parameters are in a set of units that are not intuitive: hundredths of a fifteenth of a degree (1/1500) of latitude or longitude.

In the image above, all of the objects were chosen to have offsets of "9" for both offsets.

This translates to 0.006 degrees (9/1500).

Putting 100 here will get you 0.0666 degrees, or four minutes.

The maximum value you can have here is 99*99=9801, which translates to 6.534 degrees.

Of course, since this is in degrees of latitude and longitude, not distance, the actual size of the object in miles or feet depends on where you are on the earth. Near the equator, setting this value to "9" for a rectangle corresponds to about 2160 feet in both dimensions. Near my home the same object with the same offsets is about 1792 feet wide and about 2190 feet high.

But wait, there's more

Worse, the value you enter is not actually the value that gets transmitted in the packet; the value transmitted is the integer part of the square root of what you enter, dramatically limiting the range of sizes you actually can choose here to perfect squares (integers that are the square of smaller integers).

If you enter "9" APRS requires that the packet be transmitted with a "3". If you enter 15, well, it still gets transmitted with a "3" because the integer part of the square root of 15 is 3.

I said it wasn't intuitive.

Xastir does not display area objects as specified in the APRS spec

According to the spec, the point you choose for the object is supposed to be the upper left corner of the area (or in the case of circles, the bounding box of the circle) and the offsets are supposed to apply down and to the right, but because it was claimed back in 2002 that APRSDOS (the original APRS program) did not in fact do that, neither does Xastir.

Instead, Xastir draws the object up and to the left of the specified point, or circles with the specified point at the center. Thus, area objects you create and transmit will appear in different orientations when viewed with other APRS clients that actually display area objects (many do not) and do so according to the spec.

Nobody noticed or reported this incompatibility for 24 years until we started drafting this documentation on Github, checking spec compliance, and testing Xastir against other clients. It is our intent to correct it in a future release.

The "Line right" and "Line left" objects can have a "corridor"

The line left and line right area objects define a line that extends up by the amount you specify for the "Offset Up" and left (or right) by the second offset. These offsets are also 1/1500 of a degree (but no matter what you enter, it'll be reduced to the nearest perfect square number below that).

These objects can also have a "corridor" specified in miles that will extend in both directions to either side of the line.

Some APRS clients appear not to bother implementing area objects

Area objects aren't well supported in APRS clients. The web site aprs.fi doesn't display them at all (it only displays location of the point you put in, which it just displays with a default marker, and doesn't show the area described by the object). APRSDroid does the same limited display as aprs.fi. YAAC supports them perfectly well and draws the areas down and to the left according to the spec.

Coupled with the fact that these objects are hard to get sized right and that Xastir doesn't really display them at the correct orientation that the spec says they should be, area objects probably don't have much use unless you're only communicating with other Xastir users who see them just as you created them.