CODEWITHSUNDEEP
mysqlgeospatial
user1641165
user1641165

Reputation: 456

MySQL Spatial Simplify Geometries

I'm trying to create a web map which contains large polygon data-sets. In order to increase performance, I'm hoping to decrease polygon detail when zooming out.

Is MySQL able to simplify polygons and other geometries as part of a query?

Upvotes: 3

Views: 2365

Answers (3)

John Powell
John Powell

Reputation: 12571

EDIT: As James has pointed out, since 5.7, MySQL does support ST_Simplify.

I am afraid there is no simplify function in MySQL spatial. I filed that as a feature request five years ago, and you can see it has received no attention since then.

You have a number of options depending on whether you want to do this as a one off, or if you have dynamic data.

1). Write a quick and dirty function using the PointN function to access your points, only taking every 5th point say, creating a WKT string representing a simplified geometry, and then recreating a geometry using the GeomFromText function.

2). Dump your polygon as WKT, using AsText(geom) out to csv. Import into Postgres/Postgis using COPY command (equivalent of LOAD DATA INFILE), and use the ST_Simplify function there, then reverse the process to bring in back into MySQL.

3). Use ogr2ogr to dump to shp format, and then a tool like mapshaper to simplify it, output to shp and then import again using ogr2ogr. Mapshaper is nice, because you can get a feel for how the algorithm works, and could potentially use it to implement your own, instead of option 1.

There are other options, such as using Java Topology Suite if you are using Java server side, but I hope this gives you some idea of how to proceed.

I am sorry that the initial answer is, No. I went through this a number of years ago and eventually made a permanent switch to Postgres/Postgis, as it is much more fully featured for spatial work.

Upvotes: 6

Alexander van Oostenrijk
Alexander van Oostenrijk

Reputation: 4764

I had a use case where I wanted to use ST_Simplify, but the code had to run on MySQL 5.6 (which doesn't have it). Therefore, I developed a solution like the one suggested by John Powell in another answer.

MySQL does not unfortunately offer any aggregates, whereby you can create geometry by progressively adding points to it (i.e. there is no ST_AddPoint or similar). The only way you can compose a geometry is by building it step-by-step as a WKT string, then finally converting the completed string to a geometry.

Here is an example of a stored function that accepts a MultiLineString, and simplifies each LineString in it by only keeping every nth point, making sure that the start and end points are always kept. This is done by looping through the LineStrings in the MultiLineString, then through the points for each (skipping as required), and accumulating the lot in a WKT string, which is finally converted to a geometry using ST_GeomCollFromText.

-- geometryCollection: MultiLineString collection to simplify
-- skip: Number of points to remove between every two points
CREATE FUNCTION `sp_CustomSimplify`(gc geometrycollection, skip INT) RETURNS geometrycollection
BEGIN
  DECLARE i, j,numLineStrings, numPoints INT;
  DECLARE ls LineString;
  DECLARE pt, lastPt Point;
  DECLARE ls LineString;
  DECLARE lastPt Point;
  DECLARE txt VARCHAR(20000);
  DECLARE digits INT;
  SET digits = 4;

  -- Start WKT string:
  SET txt = 'MULTILINESTRING(';

  -- Loop through the LineStrings in the geometry (which is a MultiLineString)
  SET i = 1;
  SET numLineStrings = ST_NumGeometries(gc);
  loopLineStrings: LOOP
    IF i > numLineStrings THEN LEAVE loopLineStrings; END IF;
    SET ls = ST_GeometryN(gc, i);

    -- Add first point to LineString:
    SET pt = ST_StartPoint(ls);
    SET txt = CONCAT(txt, '(', TRUNCATE(ST_X(pt),digits), ' ', TRUNCATE(ST_Y(pt),digits));

    -- For each LineString, loop through points, skipping
    -- points as we go, adding them to a running text string:
    SET numPoints = ST_NumPoints(ls);
    SET j = skip;
    loopPoints: LOOP
      IF j > numPoints THEN LEAVE loopPoints; END IF;
      SET pt = ST_PointN(ls, j);
      -- For each point, add it to a text string:
      SET txt = CONCAT(txt, ',', TRUNCATE(ST_X(pt),digits), ' ', TRUNCATE(ST_Y(pt),digits));
      SET j = j + skip;  
    END LOOP loopPoints;

    -- Add last point to LineString:
    SET lastPt = ST_EndPoint(ls);
    SET txt = CONCAT(txt, ',', TRUNCATE(ST_X(lastPt),digits), ' ', TRUNCATE(ST_Y(lastPt),digits));

    -- Close LineString WKT:
    SET txt = CONCAT(txt, ')');
    IF(i < numLineStrings) THEN
      SET txt = CONCAT(txt, ',');
  END IF;

    SET i = i + 1;
  END LOOP loopLineStrings;

  -- Close MultiLineString WKT:
  SET txt = CONCAT(txt, ')');


  RETURN ST_GeomCollFromText(txt);
END

(This could be a lot prettier by extracting bits into separate functions.)

Upvotes: 1

James
James

Reputation: 3709

MySQL 5.7 contains ST_Simplify function for simplifying geometries.

From https://dev.mysql.com/doc/refman/5.7/en/spatial-convenience-functions.html:

ST_Simplify(g, max_distance)

Simplifies a geometry using the Douglas-Peucker algorithm and returns a simplified value of the same type, or NULL if any argument is NULL.

MySQL 5.7 includes a huge overhaul of spatial functionality compared to 5.6 and is now in General Availability status as of 5.7.9.

Upvotes: 5

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