Truncate and simplify generates Error: invalid polygon

Latest turfjs version, location: /@turf/turf@6.5.0/turf.min.js

Code for reproducing:

const polygon =  {"type":"Feature","properties":{},"geometry":{"type":"Polygon","coordinates":[[[12.56231543067888,55.71174271303183],[12.5622924993585,55.7117097183066],[12.56245909105533,55.71165693489258],[12.56248995363732,55.71168436284357],[12.56231543067888,55.71174271303183]]]}} 
turf.truncate(polygon)
turf.simplify(turf.truncate(polygon, { precision: 4 }))

Error generated:

turf.min.js:43 Uncaught Error: invalid polygon
    at en (turf.min.js:43)
    at turf.min.js:43
    at Array.forEach (<anonymous>)
    at tn (turf.min.js:43)
    at turf.min.js:88
    at turf.min.js:88
    at q (turf.min.js:1)
    at Object.t.simplify (turf.min.js:88)
    at <anonymous>:3:6

After debugging, it is the simplify function that removes duplicates, and thus ends up with a polygon that is not a polygon anymore.

[JamesLMilner]

Closing as there has been no follow up. Hope this helped!

[JamesLMilner]

Hey @kevinsimper, no problem at all. Appreciate you providing more details always helpful to gain context.

Here, I can totally appreciate that it is a surprise that truncate produces a Polygon which is invalid for the simplify function. However, here as the developer you are in control of the precision parameter passed to truncate. If we think about what truncate does, it cuts off the precision of the geometries coordinates to a given precision, in this case you have chosen 4. Here, a coordinate like:

[12.56231543067888, 55.71174271303183]

becomes:

[12.5623, 55.7117]

so the actual behaviour is exactly correct. This is to say, truncating the coordinates you have given as example to a precision of 4 produces exactly the right output (there is no bug in the behaviour). Think about it as per JavaScript’s toFixed number method, if you ran:

const float = 12.56231543067888;
const fixedPrecisionStr = float.toFixed(4);
// fixedPrecisionStr is 12.5623 here

It’s the same logic to truncate in this scenario.

As a developer you can control the precision to ensure that truncate does not produce an invalid Polygon input for the simplify function. For example, you can loop incrementing the precision until it produces a valid Polygon for your requirements.

In the interests of closing this, I’ve written a solution that should do the above:

let success = false;
let precision = 4;

while (!success) {
  let truncatedPolygon = truncate(polygon, { precision });
  try {
    let simplifiedPolygon = simplify(truncatedPolygon);
    success = true;
  } catch (error) {
    precision++;
    if (precision > 11) {
      throw Error("Could not make polygon valid");
    }
  }
}