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Editorial Team
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Asked: 2026-05-29T13:56:01+00:00

I’m trying to merge a new encoded polyline with an existing polyline without decoding

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I’m trying to merge a new encoded polyline with an existing polyline without decoding and reencoding the whole polyline. The new encoded polyline will be uploaded to a (linux) server where I would like to append it to the existing polyline.

The problem is, you can’t just mash them together. Below is some sample data to play with. My hope is to find/create a solution in either PHP or a shell script but the problem is, I have no where near enough technical understanding to interpret the encoded polyline algorithm. 

41.386692,-73.475912
41.424822,-73.375027
41.428292,-73.311173
41.426183,-73.254577
41.470168,-73.218532
41.498865,-73.155278
(Yes, 6 points are easy, but it's going to be more like 7,000 coordinate pairs)
  • First 3 Coordinate Pairs Encoded: yir{Fnwm_MimFquRuTanK
  • Last 3: s`z{Fbpb~L{qGg`FkrDkjK
  • All 6: yir{Fnwm_MimFquRuTanKdLw`J{qGg`FkrDkjK

Interactive Polyline Encoder Utility
Encoded Polyline Algorithm Format (you can get to this via Interactive Encoder)
Polyline Encoder

Edit:

I also have the original data that encoded the polylines on both ends. So I can also save the first and last coordinate pair separately.

Helpful Reads:

I ended up writing a blog post that has a lot more detail about how encoded polylines work. You can read it here: What is an Encoded Polyline?

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1 Answer

  1. Editorial Team

    Ok, so I think I figured it out. Many thanks to Andrew Leach for explaining how the algorithm actually works in plain english.

    Problem: Merging a new encoded polyline with an existing encoded polyline

    Solution: keep last coord pair from existing polyline, encode just that pair and save it for later, encode all new coords with coord from existing polyline at the beginning of this new encoding. find the string of the last coordinate pair, and remove it from the new encoded polyline and stick the new encoded polyline onto the back of the existing polyline

    Things to know: What the encoding does is calculate the offset (distance from x,y) and converts that value to ASCII. The problem is the first coordinate is calculated from 0,0 so if you were just to put two encoded polylines together, where you added the new one wouldn’t be offset from the existing, but offset from 0,0 causing a large jump in the polyline. What we need to do is find out which characters in the encoded polyline are the offset from 0,0 and remove them. Then you can append the new line to the old line, and it will be offset properly.

    Click the link below to see it all written up and with good comments. Also, please let me know if you see anywhere the efficiency can be improved!

    PasteBin: PHP implementation of solution

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