PyGarmin - A Python Interface to Garmin GPS Equipment

Here's the important link:

• PyGarmin Project Page at SourceForge

Introduction

PyGarmin is a set of Python classes which implement the protocol used by Garmin GPS receivers to talk to each other and to other machines. It is based on the official protocol specification. The project was started by Quentin Stafford-Fraser but several others have helped to make it what it is today.

PyGarmin is not a complete application. Some simple applications are now included, one of which is called pygarmin, but it is primarily just a toolkit to help you write applications. (I'm assuming you know how to program in Python. If you don't, check it out. It won't take you long to learn, and is well worth it.) This is a project which is in development. No support. No guarantees. And so forth.

Having said all of that, this has been used to transfer information to and from several different Garmin receivers, mostly under Linux, though there is some Windows support now and people have used it on Mac OS X as well. If you use PyGarmin, it will probably be much quicker than writing your own software from scratch. If it works on your GPS, let us know. If it doesn't, let us know. The more info you can give us about what went wrong, the more likely we are to fix it. The best place to discuss issues with PyGarmin is on the mailing list.

If you want to cut straight to the code go to the SourceForge project page mentioned above. But I suggest you read on first. The code looks quites scary if you don't know what's happening!

Basics

Almost every model of Garmin receiver implements a slightly different protocol. They have many things in common, but there are minor differences. For example, some receivers can display icons, and they therefore transmit waypoints which have an extra 'symbol' field, which is not used in other models. Others don't use icons, but do store altitude. And so forth. You need to get the protocol right for your particular model.

This makes matters more complicated, but at least these things are well documented by Garmin. The specification includes a big table which details, for each product type, what protocol it uses for basic commands, what it uses for downloading waypoints, what it uses for downloading routes, and so forth.

I have created Python classes for each of the protocols listed in the spec, and for each of the data types. Well, most of them. The big table becomes, in Python, a mapping from the Garmin product ID to the set of relevant classes. This means that, while there are a large number of classes defined in the source, only a few of them will ever be used by any given receiver. The classes are all given names based on those used in the specification, so look at the spec if you want to know more about the classes.

The class garmin.Garmin will connect to your GPS, read its product ID and software version, and then look up the appropriate classes in the table. It creates instances of the protocol classes and notes the datatype classes for each type of data used in the transmisisons. It also has some friendly methods like 'getWaypoints', which do what you would expect. What you get back when you call this is a list of objects, each of which is an instance of a class derived from garmin.Waypoint, but the precise type of the objects will depend on the GPS you're talking to.

OK. Here's a simple Python program. You may need to set suitable permissions on the serial port (e.g /dev/ttyS0) before running it.

#! /usr/local/bin/python
# Load the module

import garmin

# Create a 'physical layer' connection using serial port

phys = garmin.UnixSerialLink("/dev/ttyS0")

# Create a Garmin object using this connection

gps = garmin.Garmin(phys)

# Get the waypoints from the GPS
# (This may take a little while)

waypoints = gps.getWaypoints()

# Print the waypoints

for w in waypoints:
    print w.ident,
    lat = garmin.degrees(w.slat)
    lon = garmin.degrees(w.slon)
    print lat, lon, w.cmnt

Simple, eh? This should work for almost any model, because all waypoints will have an identity, a latitude & longitude, and a comment field. The latitude and longitude are stored in 'semicircle' coordinates (basically degrees, but scaled to fill a signed long integer), and so the fields are called 'slat' and 'slon'. The function garmin.degrees() converts these to degrees.

More details

There are 3 levels of protocol documented:

The specification documents the various different versions of these under labels of Pxxx, Lxxx, Axxx etc, where xxx is a number, and this convention is followed here. There are also various data types, named Dxxx. Roughly speaking, the Physical protocols specify RS232, the Link protocols specify a packet structure for sending messages to and fro, and the Application protocols specify what can actually go in those packets.

For example, a Garmin GPS 38 will talk to your computer over physical layer P000 (RS232) using a packet structure defined by link layer L001. If you want to transfer waypoints to and from it, they will be sent using application layer A100 (a waypoint transfer protocol), and the actual waypoints transferred will be of type D100.

At the time of writing, the only documented physical layer is P000 which is roughly RS232 at 9600 baud, 8 data bits, no parity, 1 stop bit. In the software, we model this as a P000 class that has read and write methods, which can be used by the higher protocol levels. The UnixSerialPort class used in the sample code above is a subtype of P000.

I hope that's enough to get you going.

Some data type classes are not implemented here, just because I got bored of typing. We've done the ones used by the more common units, but if yours isn't covered, it should be easy to add. They're only a few lines each.

Acknowledgements

Thanks are due particularly to James A. H. Skillen, who implemented many of the more recent bits of the protocol, and so made PyGarmin useful for people with newer GPS units than mine! And to the gradually increasing group of other developers who are listed here. All of their contributions are most welcome.

Download

This software is released under the GNU General Public Licence. It comes with no warranties, explicit or implied, and you use it at your own risk. It may be Y2K compliant.

You can download the releases here, or you can get the up-to-date versions via CVS from the SourceForge site.

For Debian users, there is also an apt repository available. Add the following line to your apt.sources list.

# Python Interface to Garmin GPS Equipment

deb http://pygarmin.sourceforge.net debian/
deb-src http://pygarmin.sourceforge.net debian/

Quentin Stafford-Fraser