Mapping Hacks

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Chapter 5, Mapping with Gadgets

#62 Build a Car Computer

HACK

Regardless of what operating system you choose, you can always have fun

writing custom code to integrate with your mobile computer. When start-

ing on a project like this, some of the newer high-level languages can make

for a rapid and painless development experience. Python is a perfect exam-

ple of this and, as you will see in the following examples, can be used to

build a visual GPS plotter with only a few pages of code.

Connecting a GPS to Your Car Computer

If you have access to a GPS with a serial interface, it is quite easy to extract

latitude and longitude into your computer. Just about all GPS devices sup-

port a protocol called NMEA-0183 (National Marine Electronics Associa-

tion), which is streamed over the serial port at 4800 baud. The NMEA-0183

protocol is text-based and quite easy to parse. You can use the following

Python code to connect with a GPS or explore gpsd, which provides a server

interface to the GPS. gpsd is described in

“Connect to Your GPS from

Multiple Applications”

[Hack #57]. NMEA consists of various sentences with

data fields separated by commas. Each line of a NMEA sentence starts with

a

$ and a sentence type. Each sentence type provides different information in

the fields that follow the sentence-type designator. Here is what a typical

stream looks like:

$GPVTG,269.6,T,250.8,M,019.9,N,0036.8,K*76

$GPRMC,011106,A,4742.1321,N,12221.2108,W,021.0,270.5,240804,018.9,E*67 ...