Python program to check if a set of points is at land or at sea
Style

Your docstring at the top of the seq function should be moved underneath the function signature, like this:

def seq(start, end, step):
    """Function to iterate with decimal step size"""
    ...


The variable map, should be written like this, to improve clarity and readability:

map = Basemap(
    projection="merc",
    resolution = 'h', 
    area_thresh = 0.001,
    llcrnrlon=bottomlong, 
    llcrnrlat=bottomlat,
    urcrnrlon=toplong, 
    urcrnrlat=toplat
)


All these variables:

bottomlat = 70.390829
toplat = 70.855549
bottomlong = 27.692793
toplong =  28.728254
gridsize = 0.001


Should be renamed so they appropriately display the fact that they are constants.

BOTTOM_LAT = 70.390829
TOP_LAT = 70.855549
BOTTOM_LONG = 27.692793
TOP_LONG =  28.728254
GRID_SIZE = 0.001


This comment:

"""Divide global area into grid cells"""


Should be changed into an inline comment:

# Divide global area into grid cells


For more help with style, visit PEP8, Python's official style guide.



Error handling in seq

You shouldn't be using assert to check if a variable meets certain constraints unless you're doing testing. The proper way would be to do something like this:

if step != 0:
    sample_count = abs(end - start) / step
    return itertools.islice(itertools.count(start, step), sample_count)
raise ValueError("Step must not be equal to zero.")