Cartometric Blog

A scrapbook of GIS tricks, with emphasis on FOSS4G.

Archive for the ‘well known text’ tag

Convert Google Maps Polygon (API V3) to Well Known Text (WKT) Geometry Expression

with 7 comments

There’s dozens of reasons why you might want the Well Known Text (WKT) geometry expression for a Google Maps Polygon object.

Assuming you’re using the Google Maps API V3, and you’ve got a variable referencing your Polygon, I’ll suggest two approaches you can take to iterate over the paths and vertices in your Google Maps polygon and return the geometry expression as a Well Known Text string.

Add a Simple Utility Method to Your Project

Easy enough. Just add the following method to your project. Look below the method for an example of how you’d call it.

function GMapPolygonToWKT(poly)
{
 // Start the Polygon Well Known Text (WKT) expression
 var wkt = "POLYGON(";

 var paths = poly.getPaths();
 for(var i=0; i<paths.getLength(); i++)
 {
  var path = paths.getAt(i);
  
  // Open a ring grouping in the Polygon Well Known Text
  wkt += "(";
  for(var j=0; j<path.getLength(); j++)
  {
   // add each vertice and anticipate another vertice (trailing comma)
   wkt += path.getAt(j).lng().toString() +" "+ path.getAt(j).lat().toString() +",";
  }
  
  // Google's approach assumes the closing point is the same as the opening
  // point for any given ring, so we have to refer back to the initial point
  // and append it to the end of our polygon wkt, properly closing it.
  //
  // Also close the ring grouping and anticipate another ring (trailing comma)
  wkt += path.getAt(0).lng().toString() + " " + path.getAt(0).lat().toString() + "),";
 }

 // resolve the last trailing "," and close the Polygon
 wkt = wkt.substring(0, wkt.length - 1) + ")";

 return wkt;
}

Here’s how you’d access the Well Known Text expression using the utility method:

// Assuming you've already instantiated "myPolygon" somewhere.
var wkt = GMapPolygonToWKT(myPolygon);


Extend Google’s Polygon Object Prototype with a ToWKT() Method

There’s nothing wrong with the first approach, but you might find it handy to extend Google’s Polygon object prototype, itself, to include a ToWKT() member function, which makes it even easier to get its Well Known Text. To do that, add the following JavaScript somewhere near the top of your code (caveat—this will need to be called after the Google Maps library has been loaded):

if (typeof google.maps.Polygon.prototype.ToWKT !== 'function')
{
 google.maps.Polygon.prototype.ToWKT = function()
 {
  var poly = this;
  
  // Start the Polygon Well Known Text (WKT) expression
  var wkt = "POLYGON(";
  
  var paths = poly.getPaths();
  for(var i=0; i<paths.getLength(); i++)
  {
   var path = paths.getAt(i);
   
   // Open a ring grouping in the Polygon Well Known Text
   wkt += "(";
   for(var j=0; j<path.getLength(); j++)
   {
   // add each vertice, automatically anticipating another vertice (trailing comma)
   wkt += path.getAt(j).lng().toString() + " " + path.getAt(j).lat().toString() + ",";
   }
   
   // Google's approach assumes the closing point is the same as the opening
   // point for any given ring, so we have to refer back to the initial point
   // and append it to the end of our polygon wkt, properly closing it.
   //
   // Additionally, close the ring grouping and anticipate another ring (trailing comma)
   wkt += path.getAt(0).lng().toString() + " " + path.getAt(0).lat().toString() + "),";
  }
  
  // resolve the last trailing "," and close the Polygon
  wkt = wkt.substring(0, wkt.length - 1) + ")";
  
  return wkt;
 };
}


If you prefer the second approach, you can get the Well Known Text expression like this:

// Assuming you've already instantiated "myPolygon" somewhere.
var wkt = myPolygon.ToWKT();

Written by elrobis

June 6th, 2014 at 10:44 am

PostGIS: query all multipolygon parcels with at least one hole

without comments

I was writing some code to iterate over Well Known Text expressions for polygon features, and I decided I needed to test the most complex edge-case I could think of–multipolygon geometries where at least one of the bound polygons has a hole (i.e. an interior ring).

I ended up with the following query. This seems like the kind of thing I’ll want to reuse later, so I’m noting it here. For good measure, I also use a rudimentary technique to sort the output with the most complicated geometries in the table at the top of the list. Basically, the more “text” it takes to describe the geometry using Well Known Text, the larger and more complex I figure it must be!

SELECT
  SomePrimaryId,   /* your primary key, i.e. ogc_fid, etc. */
  SomeUniqueId,    /* your descriptive id, i.e. a parcel number */
  ST_NumGeometries(wkb_geometry) AS num_geoms,
  ST_NRings(wkb_geometry) AS num_rings,
  ST_AsText(ST_Centroid(wkb_geometry)) AS center,
  Char_Length(ST_AsText(wkb_geometry)) AS len,
  ST_AsText(wkb_geometry) AS wkt
FROM SomePolygonTable
WHERE
  ST_NumGeometries(wkb_geometry) > 1
  AND
  ST_NRings(wkb_geometry) > ST_NumGeometries(wkb_geometry)
ORDER BY Char_Length(ST_AsText(wkb_geometry)) ASC ;

 

Just for the sake of promoting caution, I’m not certain this is a definitive approach for identifying the largest geometry in a table, as the length of the binary representation and the length of the readable text representation do not correspond one-to-one. Moreover, a feature could have more vertices that required less precision to express (fewer decimal position), than a geometry with fewer vertices that needed more precision, and then you have to ask, which is bigger, fewer vertices and more text, or more vertices that coincidentally did not require as much text? My conclusion is, the “most complicated geometry” is probably relative to the one asking the question. However for my purposes, this was close enough to put the most complicated stuff at the top of the list.

Written by elrobis

November 8th, 2013 at 10:26 am

Decode Google Map encoded points as Well Known Text (WKT) with Python

with 2 comments

I had close encounter of the 5th kind yesterday.. here’s the gist..

It started when someone “gave” me a GIS dataset (..of polygons, kind of..) that a colleague of theirs, way back in ancient history, chose to pre-cook as ASCII-encoded point pairs. Their intention was almost certainly to use the pre-cooked data in Google Maps. Anyway, being arguably sane, I wanted to return this data to a more GIS-normal format so I could put it in a database like MySQL or Post and use it for other stuff.

I considered a few different approaches to this problem, including creating a Google Map that could load-in all of the polygons from their encodings, then iterate over the polygons, interrogate the polygon point pairs, and finally concatenate WKT features from the points and save the geofeatures into a MySQL table. This approach offered the advantage of using Google’s existing Maps API to do the decoding for me. But let’s face it, that’s lame, uninspired, and not inventive.. it wasn’t even interesting.

Besides..  I wanted to use Python.

I expected to find a Python recipe for this looming in the misty www, but I didn’t. However, I did find a JavaScript recipe by trolling around in Mark McClure’s website. Specifically, he provides a Polyline Decoder utility, and when I viewed the page source, I found the JavaScript code that actually does the decoding (opening in FireFox will show you the code, or IE should prompt you to download the file).

Long story short, the following Python method is an adaptation of Mark McClure’s JavaScript method (twisted a little to return WKT features rather than the pure point array). If you’re somewhat comfortable with Python, you should be able to copy/paste the method right into your Python file and start calling it; just pass-in the encoded point string and let the method do the rest.

Best / Elijah

———

def decodeGMapPolylineEncoding(asciiEncodedString):
    print "\nExtrapolating WKT For:"
    print asciiEncodedString

    strLen = len(asciiEncodedString)

    index = 0
    lat = 0
    lng = 0
    coordPairString = ""

    # Make it easy to close PolyWKT with the first pair.
    countOfLatLonPairs = 0
    firstLatLonPair = ""
    gotFirstPair = False

    while index < strLen:
        shift = 0
        result = 0

        stayInLoop = True
        while stayInLoop:                                                # GET THE LATITUDE
            b = ord(asciiEncodedString[index]) - 63
            result |= (b & 0x1f) << shift
            shift += 5
            index += 1

            if not b >= 0x20:
                stayInLoop = False

        # Python ternary instruction..
        dlat = ~(result >> 1) if (result & 1) else (result >> 1)
        lat += dlat

        shift = 0
        result = 0

        stayInLoop = True
        while stayInLoop:                                                # GET THE LONGITUDE
            b = ord(asciiEncodedString[index]) - 63
            result |= (b & 0x1f) << shift
            shift += 5
            index += 1

            if not b >= 0x20:
                stayInLoop = False

        # Python ternary instruction..
        dlng = ~(result >> 1) if (result & 1) else (result >> 1)
        lng += dlng

        lonNum = lng * 1e-5
        latNum = lat * 1e-5
        coordPairString += str(lonNum) + " " + str(latNum)

        if gotFirstPair == False:
            gotFirstPair = True
            firstLatLonPair = str(lonNum) + " " + str(latNum)

        countOfLatLonPairs += 1

        if countOfLatLonPairs > 1:
            coordPairString += ","

    # The data I was converting was rather dirty..
    # At first I expected 100% polygons, but sometimes the encodings returned only one point.
    # Clearly one point cannot represent a polygon. Nor can two points represent a polygon.
    # This was an issue because I wanted to return proper WKT for every encoding, so I chose
    # To handle the matter by screening for 1, 2, and >=3 points, and returning WKT for
    # Points, Lines, and Polygons, respectively, and returning proper WKT.
    #
    # It's arguable that any encodings resulting in only one or two points should be rejected.
    wkt = ""
    if countOfLatLonPairs == 1:
        wkt = "POINT(" + coordPairString + ")"
    elif countOfLatLonPairs == 2:
        wkt = "POLYLINE(" + coordPairString + ")"
    elif countOfLatLonPairs >= 3:
        wkt = "POLYGON((" + coordPairString + "," + firstLatLonPair + "))"

    return wkt

Written by elrobis

October 20th, 2012 at 1:00 pm