Python Basemap Coordinates

1
votes

I am using python 3.6 to open a shapefile of the Amazon River on to basemap. However I am confused with how coordinates work in python. I looked up coordinates of the the Amazon River and found it to be lon,lat=-55.126648,-2.163106. But to open my map I need the lat/lon values of corners, which I am not sure how to get.

Here is my code so far:

from mpl_toolkits.basemap import Basemap

import matplotlib.pyplot as plt

map= Basemap(projection='tmerc',
            lon_0=180,
            lat_0=0,
            resolution='l')
map.drawmapboundary(fill_color='aqua')
map.fillcontinents(color='#ddaa66',lake_color='aqua')
map.drawcoastlines()

map.readshapefile('filename','Amazon')
plt.show()

Here is the error message I get when I try to run it: ValueError: must either specify lat/lon values of corners (llcrnrlon,llcrnrlat,ucrnrlon,urcrnrlat) in degrees or width and height in meters

2
pythoncoordinatesshapefilematplotlib-basemap

2 Answers

0
votes

When creating your map (map = Basemap(...)) , you need to specify those values. They are lower left corner longitude, lower left corner latitude, upper right corner longitude, and upper right corner latitude. These define the extents of the map. You could just plot the whole earth, then look at the region you want and pick the points off of it for your new corners.

0
votes

The best method for this type of point plotting is to create your own corners by 'zooming out' from the point. this means you'll need to specify llcrnrlat (lower left corner latitude), etc. as such:

my_coords = [38.9719980,-76.9219820]

# How much to zoom from coordinates (in degrees)
zoom_scale = 1

# Setup the bounding box for the zoom and bounds of the map
bbox = [my_coords[0]-zoom_scale,my_coords[0]+zoom_scale,\
        my_coords[1]-zoom_scale,my_coords[1]+zoom_scale]

plt.figure(figsize=(12,6))
# Define the projection, scale, the corners of the map, and the resolution.
m = Basemap(projection='merc',llcrnrlat=bbox[0],urcrnrlat=bbox[1],\
llcrnrlon=bbox[2],urcrnrlon=bbox[3],lat_ts=10,resolution='i')

If you want to see a full tutorial on plotting lat/lon points from a .csv file, check out my tutorial where I go through the whole process and include the full code:

Geographic Mapping from a CSV File Using Python and Basemap

You end up with a result that looks like the following:

enter image description here