Extracting List of coordinates from string

Question

Hello Iv been at this all day but have ran into a problem.

I am reading in GeoJson file and trying to extract the coordinates so that I can later iterate throughout them to determine the size of my window to redraw the polygons, represented by a list of coordinates. Each polygon may have 4 - x coordinate pairs. When I read in the GeoJson I get a list containing only 1 element for each polygon instead of a list with a list of coordinates. I believe this is because the list reads: [[[-76.2671328, 38.4506304], [-76.2669856, 38.4505256], [-76.2668385, 38.4503701], [-76.2667281, 38.4502182], [-76.2664568, 38.4499759], [-76.2661993, 38.4497843], [-76.2660108, 38.4497192]...]] and there are multiple sets of [[]] which make the list think there is one element inside..?

So then i tried converting the list of coordinate to a string so I could remove the extra [[]] and extract the coordinate pairs and turn them back into a list. Here is the code:

import json
import re

with open('landareas.json') as f:
    landareas = json.load(f)

coordinates = []
polygons = []
for feature in landareas['features']:
    polygons.append(feature['geometry']['coordinates'])

print len(polygons)
for polygon in polygons:
    #print type(polygon)
    #print len(polygon)
    string =  str(polygon)  
    newCords = string[2:len(string)-1]
    print newCords
    coordinates.append(re.findall('[(.+?)]', newCords))

for coordinate in coordinates:
    print coordinate
    print

So now when i do this all i get back is:

['.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.', '.']

With all the numbers stripped out.. :(!

If someone could please help me out in a method to exact the coordinates into a list of coordinates for each polygon so that I can loop through a list of polygons with each containing a list of their coordinates, that would be greatly apprcieated!

My initial error which started this was:

File "polygon_LatLong_xy.py", line 33, in <module>
    for x,y in polygons[CORDINATES]:
TypeError: 'int' object is not utterable

from this python script:

import turtle as t
import json

RCID = 0
CORDINATES= 1

with open('landareas.json') as f:
    landareas = json.load(f)

polygons = []
for feature in landareas['features']:
    polygons.append([feature['properties']['RCID'], feature['geometry']['coordinates'] ])

map_width = 400
map_height = 400

minx = 180
maxx = -180
miny = 90
maxy = -90

for x,y in polygons[CORDINATES]:
    if x < minx: minx = x
    elif x > maxx: maxx = x
    if y < miny: miny = y
    elif y > maxy: maxy = y

dist_x = maxx - minx
dist_y = maxy - miny

x_ratio = map_width / dist_x
y_ratio = map_height / dist_y

def convert(point):
    lon = point[0]
    lat = point[1]
    x = map_width - ((maxx - lon) * x_ratio)
    y = map_height - ((maxy - lat) * y_ratio)
    #Python turtle graphics start in the middle of the screen 
    #so we must offset the points so the are centered
    x = x - (map_width/2)
    y = y -(map_height/2)
    return [x,y]

t.up()
first_pixel = None
for point in polygons[CORDINATES]:
    pixel = convert(point)
    if not first_pixel:
        first_pixel = pixel
    t.goto(pixel)
    t.down()
t.goto(first_pixel)
t.write(str(polygons[RCID]), align="center", font=("Arial",16,"bold"))
t.up()
t.done()

If its any help here is an excerpt from the geoJson file:

{
  "type": "FeatureCollection", 
  "features": [
    {
      "geometry": {
        "type": "Polygon", 
        "coordinates": [ <--- note the 
          [ <----------------two sets of brackets? could this be why?              
            [
              -76.0220181, 
              38.1321203
            ], 
            [
              -76.0219133, 
              38.1321847
            ], 
            [
              -76.0232178, 
              38.1312463
            ], 
            [
              -76.0230198, 
              38.1312923
            ], 
            [
              -76.0220181, 
              38.1321203
            ]
          ]
        ]
      }, 
      "type": "Feature", 
      "properties": {
        "TXTDSC": "                                                                                ", 
        "RCID": 3918, 
        "PRIM": 3, 
        "NINFOM": "                                                                                ", 
        "SORIND": "                                                                                ", 
        "RECDAT": "                                                                                ", 
        "AGEN": 550, 
        "GRUP": 1, 
        "SORDAT": "                                                                                ", 
        "OBJL": 71, 
        "NOBJNM": "                                                                                ", 
        "INFORM": "                                                                                ", 
        "LNAM": "0226088C104B1046", 
        "STATUS": "                                                                                ", 
        "RECIND": "                                                                                ", 
        "SCAMAX": null, 
        "NTXTDS": "                                                                                ", 
        "CONDTN": null, 
        "FIDS": 4166, 
        "SCAMIN": null, 
        "FIDN": 143396939, 
        "RVER": 1, 
        "OBJNAM": "                                                                                "
      }
    },

Also, you should look into why the system thought polygons[CORDINATES] was an integer instead of an array. I made a theory about what was wrong in my answer, but that could explain a lot about what is going on :). — ytpillai
I agree and that was how i wound up converting what should have been a list of coordinate pairs into a string with coordinate pairs ha — James Laney
OK, so after you make separate strings of coordinate pairs. Take out the parentheses by str = str.replace("(", "") for both parentheses and then split by the comma. You should have your lists, and not get the iterable error — ytpillai
Removing the brackets with str.replace("[", "") will result in all brackets being removed. Repeating this with the other bracket will leave me with as string of number with commas after each number, which is no good for distinguishing pairs. [[[-76.2671328, 38.4506304], [-76.2669856, 38.4505256], [-76.2668385, 38.4503701]]] into -76.2671328, 38.4506304, -76.2669856, 38.4505256, -76.2668385, 38.4503701 but I guess I could str.replace("],", "|") or another charter to distinguish pairs and then spitting on the same charter. — James Laney
Ah, your problem is that you have everything in one string. Alright this is what you do. First, you need to make a string array by splitting the string by ",". THEN do the replacing. — ytpillai

ytpillai ytpillai · Accepted Answer · 2015-06-30T22:50:44

I'm not complete sure about your code, but I think your problem is that when you get the string value of the coordinates (in the polygons.append(feature['geometry']['coordinates']) and coordinates.append(re.findall('[(.+?)]', newCords)), you don't convert them into a list first.

Let's say we have a string c = "1, 2" You can do biglist.append(c.split(","))

One more thing that might help is that you make a set of coordinates for each polygon.

#Make a condition that tells the program when to search for the next polygon
polygons = []
polygon = []
for x in landareas['features']:
    if polygonDone:
        polygons.append(polygon)
        del polygon[:]
    else:
        polygon.append(x['geometry']['coordinates'].split(","))

Extracting List of coordinates from string

2 Answers