[talk-ph] better ways to coordinate coastline mapping?

Eugene Alvin Villar seav80 at gmail.com
Tue Jul 6 19:03:41 BST 2010


On Tue, Jul 6, 2010 at 2:42 PM, Jim Morgan <jim at datalude.com> wrote:

> Eugene Alvin Villar wrote, On Tuesday, 06 July, 2010 11:57 AM:
> > To clarify things, the sawtooth detection script is quite naive. It
> > simply detects if there are at three or more series of nodes where each
> > pair of adjacent nodes have the same latitude or longitude. This will
> > also detect any three linear nodes that all have the same latitude or
> > longitude like this: o----o----o
>
> There are a number of cases where a near-straight line is acceptable. Maybe
> it would be better -- and I'm not sure if this is possible -- to examine,
> say, a series of three nodes. It would check if the first two have the same
> lat or long. If they have the same lat, then the second and third points
> would need the same long; if they have the same long, then the second and
> third points would need the same lat. Then you'd be correctly identifying
> the step-fashion jaggies, rather than straight lines.
>
>
> To increase certainty, you could make this a series of four, or five
> points. Again I don't know if this is possible or plausible, but it would
> seem like a better pattern to look for. Not sure if the formatting will come
> through but ....
>
>
>  p1 |_____ p2
>          |
>          |
>       p3 |______  p4
>                  |
>

I intentionally wanted to detect collinear nodes since I wasn't sure if the
original SRTM-based data have those collinear nodes or not.

In any case, the script detects sawtooth coasts if the latitude or the
longitude is *exactly* the same, right down to the 7th decimal place (which
translates to an accuracy of about 1 cm). So if there are a series of
coastline nodes that have the same latitude or longitude for each adjacent
pair of nodes, then they are most likely generated from raster data, like
SRTM. I don't think Mother Nature created coasts that follow latitudes and
longitudes. :-)
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