Basemap datum assumption

Hello all,

I have a quick question regarding the datum assumption when entering

lon/lat data into a Basemap object.

My understanding is that Basemap will use the datum specified upon

creation, the default being a sphere of radius 6,370,997 meters and
assume that the lon, lat entered in a call follows the same
convention. Then, if you have lon lat which uses a different datum
to the projection of the basemap you must convert this to lon/lat in
the correct datum before then converting to projection coordinates
using the basemap object. Is that correct?

Assuming my understanding above is correct, I don't feel that this

process is made clear in the documentation for basemap and I’m sure
many make the mistake of thinking of lon/lat as an absolute across
all geodetic standards. I know ignorance is not an excuse, but it
might be nice if people got a more explicit warning to take care
with their inputs.

For reference, I found this blog which describes the datum mistake


Kindest regards,



Dr. Mathew Topper

  Institute for Energy Systems

  School of Engineering

  The University of Edinburgh

  Faraday Building

  The King’s Buildings

  Edinburgh EH9 3JL

  Tel: +44 (0)131 650 5570

  School fax: +44 (0)131 650 6554