Polyline encoding is a lossy compression algorithm that allows you to store a series of coordinates as a single string. Point coordinates are encoded using signed values. If you only have a few static points, you may also wish to use the interactive polyline encoding utility.
The encoding process converts a binary value into a series of character codes for ASCII characters using the familiar base64 encoding scheme: to ensure proper display of these characters, encoded values are summed with 63 (the ASCII character '?') before converting them into ASCII. The algorithm also checks for additional character codes for a given point by checking the least significant bit of each byte group; if this bit is set to 1, the point is not yet fully formed and additional data must follow.
Additionally, to conserve space, points only include the offset from the previous point (except of course for the first point). All points are encoded in Base64 as signed integers, as latitudes and longitudes are signed values. The encoding format within a polyline needs to represent two coordinates representing latitude and longitude to a reasonable precision. Given a maximum longitude of +/- 180 degrees to a precision of 5 decimal places (180.00000 to -180.00000), this results in the need for a 32 bit signed binary integer value.
Note that the backslash is interpreted as an escape character within string literals. Any output of this utility should convert backslash characters to double-backslashes within string literals.
The steps for encoding such a signed value are specified below.
- Take the initial signed value:
- Take the decimal value and multiply it by 1e5, rounding the result:
- Convert the decimal value to binary. Note that a negative value must be
calculated using its
two's complement by inverting the binary value and adding one to the result:
00000001 00010010 10100001 11110001
11111110 11101101 01011110 00001110
11111110 11101101 01011110 00001111
- Left-shift the binary value one bit:
11111101 11011010 10111100 00011110
- If the original decimal value is negative, invert this encoding:
00000010 00100101 01000011 11100001
- Break the binary value out into 5-bit chunks (starting from the right hand side):
00001 00010 01010 10000 11111 00001
- Place the 5-bit chunks into reverse order:
00001 11111 10000 01010 00010 00001
- OR each value with 0x20 if another bit chunk follows:
100001 111111 110000 101010 100010 000001
- Convert each value to decimal:
33 63 48 42 34 1
- Add 63 to each value:
96 126 111 105 97 64
- Convert each value to its ASCII equivalent:
The table below shows some examples of encoded points, showing the encodings as a series of offsets from previous points.
Points: (38.5, -120.2), (40.7, -120.95), (43.252, -126.453)
|Latitude||Longitude||Latitude in E5||Longitude in E5||Change In Latitude||Change In Longitude||Encoded Latitude||Encoded Longitude||Encoded Point|