On Tue, Sep 9, 2008 at 2:02 AM, Eric Firing <[EMAIL PROTECTED]> wrote:
> Evan Mason wrote:
>
>> Hi, wonder if anyone can help me with path and bbox. I have a set of
>> ocean drifter tracks and I want to know if they pass through a particular
>> boxed area. This is straightforward to do but I wanted to try to do it with
>> matplotlib.transforms and matplotlib.path, which look well-suited to this
>> kind of task using 'intersects_bbox'.
>>
>> The essence of my code is below. I have a single drifter track (lon, lat)
>> and have defined a boxed area. At no time does the drifter approach or
>> enter the box - however, the final line,
>>
>> print track.intersects_bbox(bbox)
>>
>> returns True, which it shouldn't. In total I have 340 tracks. My code
>> says 74 pass though the box, but by counting there should be only about 9.
>> Any help appreciated, thanks, Evan
>>
>>
>> import numpy as N
>> import numpy.ma <http://numpy.ma> as MA
>> import matplotlib.transforms as BB
>> import matplotlib.path as PP
>>
>> In [200]: lon
>> Out[200]:
>> masked_array(data = [-15.52 -15.521 -15.541 ..., -- -- --],
>> mask = [False False False ..., True True True],
>> fill_value=1e+20)
>>
>> In [201]: lat
>> Out[201]:
>> masked_array(data = [29.2 29.2 29.196 ..., -- -- --],
>> mask = [False False False ..., True True True],
>> fill_value=1e+20)
>>
>> In [202]: len(lon), len(lat)
>> Out[202]: (3750, 3750)
>>
>> track = MA.transpose([lon, lat])
>> track = PP.Path(track)
>> bbox = BB.Bbox.from_extents(-15.95, 29.6, -15.9, 29.65)
>>
>> In [206]: print track.intersects_bbox(bbox)
>> 1
>>
>
> Evan,
>
> Can you strip the problem down more? That is, find one track that is
> misdiagnosed, and then try to find the shortest segment of it that still
> gets misdiagnosed. You could try subsampling and/or chopping chunks off the
> ends. If you can get it down to a track with only a few points, so that you
> can email a very short but complete script with illustrative data, that
> would make it much easier for someone to figure out what is going on.
>
> I suspect it is related to the masked values. If you strip the masked
> points out of your lon and lat arrays, do you get the same result?
> Try making an example with only 3 points, say with the middle one masked,
> and then unmasked.
>
> Eric
>
Eric, I tried what you say about masked values, and that has helped.
However there are still a few tracks that creep through. Below is a short
script that shows what I mean. It loads in a matfile with the drifter
track, which I will send to you separately.
# %run path_example.py
import pylab as P
import numpy as N
from scipy import io
from matplotlib.transforms import Bbox as BB
import matplotlib.path as PP
P.close('all')
# Load drifter track (lon, lat)
lonlat = io.loadmat('lonlat')
lon = N.squeeze(lonlat.values()[1])
lat = N.squeeze(lonlat.values()[0])
# Set up box
box = BB.from_extents(-16.6, 28.05, -15.65, 28.15)
bx = [box.xmin, box.xmax, box.xmax, box.xmin, box.xmin]
by = [box.ymin, box.ymin, box.ymax, box.ymax, box.ymin]
track_all = N.transpose([lon, lat])
track_1 = N.transpose([lon[81:295], lat[81:295]])
track_2 = N.transpose([lon[81:290], lat[81:290]])
track_all = PP.Path(track_all)
track_1 = PP.Path(track_1)
track_2 = PP.Path(track_2)
# Test if drifter passes thru box (it doesn't so should be 0)
print track_all.intersects_bbox(box), 'Should be 0'
print track_1.intersects_bbox(box), 'Should be 0'
print track_2.intersects_bbox(box), 'Should be 0'
P.figure()
P.plot(lon, lat, 'r', lw=0.3)
P.plot(lon[81:290], lat[81:290], 'g', lw=0.5)
P.plot(bx, by, 'b')
P.show()
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