[postgis-users] selection of data in WGS84 lat/lon over the noth pole

[David Fawcett]

I think that you may need to look at using a polar projection to do
these types of operations at the poles.

David.

On Mon, Sep 28, 2009 at 8:51 AM, Brian Modra <brian at zwartberg.com> wrote:
> 2009/9/28 Brian Modra <brian at zwartberg.com>:
>> 2009/9/28 Malm Paul <paul.malm at saabgroup.com>:
>>> Hi list,
>>> I would like to select data over the north pole inside a polygon of lat/lon coordinates
>>> For selection I create a polygon from theese coordinates
>>>
>>> 1) 80, -170
>>> 2) 80, 170
>>> 3) 75, 10
>>> 4) 75, -10
>>>
>>> But is seems that the objects found are from around the pole instead of over the pole, it does not find any objects over 80°
>>> Is ther a way to get around this?
>>
>> This is a wild guess, having not looked at the PostGIS code, and not
>> knowing how your data is stored, but I wonder if you need to put in
>> more coords, because depending on the prjection, the lines joining
>> those 4 points could be kind of weird.
>>
>> e.g.
>> 80 0, 80 20, 80 40, 80 50, 80 60... 80 180, 80 -160, 80 -140 ... 80 0
>
> Another question for the experts:
> what is the inside of the polygon in the above example? Is it above
> 80, or below 80, or is the polygon joined up as a straight line at 80
> degrees? I.e. with zero area?
>
> maybe a better polygon would be:
>
> 80 -170, 80 -150, 80 -130 ... 80 150, 80 170, 75 170, 75 150, 75 130
> ... 75 -150, 75 -170, 80 -170
>
>>
>>> Kind regards,
>>> Paul
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>>>
>>
>>
>>
>> --
>> Brian Modra   Land line: +27 23 5411 462
>> Mobile: +27 79 69 77 082
>> 5 Jan Louw Str, Prince Albert, 6930
>> Postal: P.O. Box 2, Prince Albert 6930
>> South Africa
>> http://www.zwartberg.com/
>>
>
>
>
> --
> Brian Modra   Land line: +27 23 5411 462
> Mobile: +27 79 69 77 082
> 5 Jan Louw Str, Prince Albert, 6930
> Postal: P.O. Box 2, Prince Albert 6930
> South Africa
> http://www.zwartberg.com/
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>