About APRS Objects and Items

As was often stated by Bob Bruninga, APRS is not intended to be a vehicle tracking system for the internet, but a real-time on-air situational awareness system. In addition to creating station reports, APRS clients can transmit additional information about the location of "things." These are "Object" and "Item" reports.

You can create objects or items in Xastir through the Map Pointer Menu, either by right-clicking in the map window or choosing the "Map Pointer Menu" option in the Map Menu.

Choosing the "Object/Item->Create" option will open the Object/Item Create dialog:

Here you can enter basic information about the object: its name, latitude and longitude, and symbol. Some object types allow you to add a speed and course and/or altitude. All objects allow you to attach a comment.

Latitude and longitude must be in integer degrees and decimal minutes. You can use the "Calc" button next to the latitude and longitude fields to access the Coordinate Calculator if you need to enter the position in some format other than degrees and decimal minutes, just as you can do in the Configure Station dialog.

If you select the option from the right-click context menu, the latitude and longitude will be filled in with the coordinates of the mouse location. You can edit these if you wish, they are pre-filled only for the convenience of being able to point at a location and use the menu as a "place an object here" capability.

The most basic object (created without checking any of the special boxes on the right) will simply appear on the map in the same way any station would.

If given a course and speed, the object will be dead-reckoned in between transmissions. Future transmissions will be at the dead-reckoned positions.

There are several special types of objects:

Signpost objects: Can have a three character "signpost text" attached. Some APRS clients will plot this text inside the signpost symbol. Xastir does not, it places that text next to the symbol.
Area Object: used to mark a circle, triangle, rectangle, or linear feature on the map.
DF Object: used to record direction finding reports.
Map View Object: Some APRS clients will recenter the map on any map view object when they are clicked.
Probability Circles: A feature added to Xastir for the benefit of Search and Rescue use, used to plot concentric zones around the object's location (typically used to represent different zones of probable search subject location, hence the name).

Selecting any of the special object types will expand the dialog to prompt for the additional information needed for that type of object. Some of these special types allow you to change the symbol, but most do not, autoselect the symbol, and grey out the option to change the symbol.

Once the data for an object is entered, you can click either "Create New Object" or "Create New Item". The object or item will be added to Xastir's database and transmitted over the air.

There is little difference between objects and items: objects are transmitted with space-padded names and items have no padding. The on-air packet format is only slightly different, but functionally they are intended to serve the same purpose. Some APRS-capable radios from Kenwood have incomplete implementations of the APRS protocol and can't decode Items well (or at all), so if your intended recipients are using such radios you might want to stick to using Objects.

The Object/Item->Modify menu will allow you to change some of the parameters of an existing object or item. You can't change the type of the object or its name, but you can change its position, comment, course, speed, etc.

Watch out if you're modifying an object that already has a course and speed

If the object you're trying to modify has been given a course and speed, Xastir will ignore any changes you make to the latitude and longitude of the object, because it is computing those from the previous course and speed and the elapsed time. This can be confusing.

There is a bug with signpost objects

If a signpost object is given an altitude and some signpost text, Xastir will incorrectly parse its packet and not recognize the signpost text as such. That text will get stuffed into the object's comment and will not be displayed on the map. The only workaround is not to give the object an altitude.