I was planning to use Pythons is_land function, to differentiate between land and sea. However, after some inspection, I have major concerns in the accuracy. I can't find anything about this in the documentation, so I hoped anyone here could help me.

I initiate the basemap with high resolution, and a low area threshhold to help differentiate:

map = Basemap(projection='merc', resolution = 'h', area_thresh=0.001)

When I try this function, it returns true even though I choose points clearly not on land:

>>> map.is_land(39.784004,3.345337)
>>> map.is_land(39.789808,3.3395)
>>> map.is_land(39.799041,3.332291)
>>> map.is_land(39.863371,3.346367)

The three last ones is out at sea.

Anyone here that has used this function, and maybe got some tips to how I can make it more accurate, or if there is some (preferably) Python substitution, or other open source programmable solution to check if a number of points is at land or not.

Thought maybe I needed to map the points first, no better:

>>> map = Basemap(projection='merc', resolution = 'h', area_thresh=0.001)
>>> lat,long = map(39.784004,3.345337)
>>> map.is_land(lat,long)
>>> lat,long = map(39.789808,3.3395)
>>> map.is_land(lat,long)
>>> lat,long = map(39.799041,3.332291)
>>> map.is_land(lat,long)
>>> lat,long = map(39.863371,3.346367)
>>> map.is_land(lat,long)

3 Answers 3


The reason why it doesn't work is because the arguments for is_land(xin, yin) are referencing the grid, and aren't latitudinal and longitudinal coordinates, they're simply the index values.

Since is_land(xin, yin) takes index values and not lat/long coordinates, you need to find out the index values on the map for the coordinates you're interested in.

map = basemap.Basemap(
lat, long = -37, 158
x,y = map(long, lat)
>>> map.is_land(x,y)

Those lat and long coordinates are off the coast of Australia, so this works. x and y don't need to be integers, they can be floats.


Notice that the arguments for map are map(long, lat).


The documentation says "Calling a Basemap class instance with the arguments lon, lat will convert lon/lat (in degrees) to x/y map projection coordinates (in meters)".

So you are passing in points around 39.789808E, 3.3395N - those are definitely on land (in Kenya). If you were hoping for points near the coast in Spain, you've just reversed the coordinates. Read the documentation again - "lon, lat".

You are using lat and long as variable names, but you are really getting X and Y (which would be closer to longitude and latitude, not latitude and longitude).

This is a very common issue in geospatial applications. It usually helps to remember its mostly Cartesian concepts, so X then Y.


I've written a small package with a function that is much faster than basemap.is_land() https://github.com/toddkarin/global-land-mask

It provides a global binary mask of land/ocean at 1 km resolution.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.