polyline-cpp

A polyline encoder/decoder for GPS coordinates. No dynamic memory allocation, so it’s safe for
embedded programming!

I made this tool because I wanted a way to store GPS Coordinates on an embedded platform with very little available memory.
My device also communicated with other devices over serial, so it was also easier to send a polyline string rather than a large list of coordinates.

To see how polyline encoding/decoding works, check out Google’s Interactive Polyline Encoder Utility.

How To Build

This project uses CMake to build. For a quick start, simply run ./setup.sh in the root directory.

How To Use

There are two different classes made available - Polyline and SimplePolyline. Polyline uses a GPSCoordinate class
(included in src/datatypes) whereas SimplePolyline just uses arrays of floats.

Polyline Class

To use the Polyline class to encode an array of GPSCoordinates into a char array:

  int buffer_size = 10;
  float latitude = 41.75368;
  float longitude = -87.97330;
  char encoded[buffer_size] = {0};
  GPSCoordinate gps_coordinates[num_coords];
  gps_coordinates[0].set_coordinates(latitude, longitude);

  Polyline polyline();  // we have a default floating point precision of 5 decimal places
  polyline.encode(gps_coordinates, num_coords, encoded);

And your polyline will be in the encoded char buffer array.

And to decode a char array into an array of GPSCoordinates:

  GPSCoordinate decoded_coordinates[buffer_size];
  polyline.decode(encoded, decoded_coordinates);

And to access the first GPSCoordinate’s latitude/longitude:

  float latitude = gps_coordinates[0].latitude();
  float longitude = gps_coordinates[0].longitude();

We can also set the precision if we want higher/lower decimal precision when we initialize the class by:

  int precision = 3;    // default precision is 5
  Polyline polyline(precision);

SimplePolyline Class

To use the SimplePolyline class to encode an array of arrays of floats into a char array:

  static const int buffer_size = 32;
  static const int num_coords = 3;
  char encoded[buffer_size] = {0};
  float coordinates[num_coords][2] = {
    {41.87376, -87.67471},
    {41.83029, -87.70492},
    {41.81212, -87.65411}
  };
  SimplePolyline polyline();
  polyline.encode(coordinates, num_coords, encoded);

And your polyline will be in the encoded char buffer array.

And to decode a char array into an array of GPSCoordinates:

  float decoded_coordinates[num_coords][2];
  int num_decoded_coords = polyline.decode(encoded, decoded_coordinates);

And to access the first gps coordinates’s latitude/longitude:

  float latitude = gps_coordinates[0][0];
  float longitude = gps_coordinates[0][1];

We can also set the precision if we want higher/lower decimal precision when we initialize the class by:

  int precision = 3;    // default precision is 5
  Polyline polyline(precision);

Compatible Platforms

Tested on a 64bit Linux machine, 64bit OS X.

There has been preliminary testing on an Arduino Mega, but the test cases aren’t quite complete and more rigorous testing needs to occur.

How To Test

After running ./setup.sh, (which also runs the tests) go to the build directory and run ./polyline_test.