[GIS] PostGIS linestring to shapefile error with shapely/fiona

Question

I have a PostGIS layer with linestring and I am trying to write the layer to shapefile using fiona and shapely.

Here is what I have so far:

from shapely.geometry import mapping,LineString
from shapely.wkt import loads,load
import fiona
from fiona.crs import from_epsg
import psycopg2
conn = psycopg2.connect("dbname='postgres',etc..) #connecting to DB
cur = conn.cursor()  #setting up connection cursor
crs = from_epsg(4269)
schema = {'geometry': 'LineString', 'properties': {'State': 'str', 'Date': 'str'}}
with fiona.open(r"routes.shp", "w", "ESRI Shapefile", schema, crs) as output:
    cur.execute("""select state,date,ST_AsText(geom) from trips;""")
    rows = cur.fetchall()
    conn.commit()
    for row in rows:
        print row
        g = loads(row[2])
        output.write({'properties':{'State': row[0], 'Date': row[1]},
        'geometry': mapping(g)})

It throws an error when it gets to:

('South Dakota', '2016-06-19', 'LINESTRING(-103.8036896 44.4258771)')
Traceback (most recent call last):
  File "C:\Users\Ralph\Documents\PythonScripts\LocationHistory.py", line 57, in <module>
    g = loads(row[2])
  File "C:\Python27\ArcGIS10.1\lib\site-packages\shapely\wkt.py", line 10, in loads
    return geom_from_wkt(data) #factory(geom)
  File "C:\Python27\ArcGIS10.1\lib\site-packages\shapely\geometry\base.py", line 55, in geom_from_wkt
    "Could not create geometry because of errors while reading input."
ReadingError: Could not create geometry because of errors while reading input.

I used the same select query in postgres on the routes table and

('South Dakota', '2016-06-19', 'LINESTRING(-103.8036896 44.4258771)') 

seems to be the error.

It writes the shapefile up to this record. what is strange is if I export the table to a shapefile using the PostGIS shapefile and dbf loader it exports the entire layer to a shapefile correctly.

My question is twofold:

How did postgis make a linestring from only one coordinate?
How would I go about circumventing this problem?

shapefile from PostGIS – this record ('South Dakota', '2016-06-19', 'LINESTRING(-103.8036896 44.4258771)') which throws the error in shapely when trying to create a linestring (because it is only 1 coordinate) shows up in the PostGIS shapefile as a record with no geometric attribute

Answer 1

The problem is that a LineString with only one point is not valid with GEOS (Shapely and many others)

With Shapely:

The LineString constructor takes an ordered sequence of 2 or more (x, y[, z]) point tuples. (from Shapely LineString)

With PostGIS:

A linestring is a path between locations. It takes the form of an ordered series of two or more points (from PostGIS intro: geometries).

Therefore a LineString with one Point gives an error:

from shapely.geometry import LineString
line = LineString([(-103.8036896,44.4258771)])
Traceback (most recent call last):
....
ValueError: LineStrings must have at least 2 coordinate tuples

You can always save your original geometry as a shapefile

import fiona
line = fiona.open("line.shp")    
line.crs
{'init': u'epsg:4269'}
first =line.next()
print first
{'geometry': {'type': 'LineString', 'coordinates': [(-103.8036896, 44.4258771)]}, 'type': 'Feature', 'id': '0', 'properties': OrderedDict([(u'FID', 0.0)])}

But you don't see anything in a GIS because

from shapely.geometry import shape
shape(first['geometry'])
....
ValueError: LineStrings must have at least 2 coordinate tuples

If you consider a Point as a Vector with an origin (point (0,0)) and an end (coordinates x, y of the point)

You can use

line = LineString([(0, 0), (-103.8036896,44.4258771)])

(with the node of the original shapefile in blue)

But don't forget that if you work with EPSG 4269 coordinates, the unit is degree (you don't work in a Cartesian plane)

Shapely does not support coordinate system transformations. All operations on two or more features presume that the features exist in the same Cartesian plane.