[Qgis-user] TIN Interpolation

[Bernd Vogelgesang]

Hi Marco,
thanx for your answer. But i fear i didn't get the message.

My main data are elevation points from an ordinary point shape with a z  
attribute.
I also digitized some peaks from a map and entered their hights in a  
different layer.

Those two point featured are handled correctly in the interpolation.

The structure lines I added are simply digitized small streams.
When adding them to the list of layers, an interpolation attribute is  
chosen from the first available numerical attribute field in that shape by  
default.

As the lines do not have any information besides their geometry, i wonder  
where i should get values from so the interpolation will work correctly.

I also tried removing all attributes,so no interpolation attribute can be  
picked by the plugin. The lines are then ignored completely.

When i give the lines some arbitratry value like 1, the interpolation  
produces real canyons, when using higher values than the surrounding area,  
it produces ridges.

So again the question: how on earth shall i be able to give those lines  
attributes values that will not destroy the interpolation.
I do not ḱnow the exact elevations of those valley grounds, which is the  
reason why i want to interpolate them!

Do i have a misconception or does the plugin?

Cheers
Bernd

p.s. Scrrenshot with structure lines with attribute value of 1:  
https://dl.dropboxusercontent.com/u/48666790/QGIS/structurelinesvalue1.png





Am 24.07.2013, 08:30 Uhr, schrieb Marco Hugentobler  
<marco.hugentobler at sourcepole.ch>:

> Hi Bernd
>
>  >So, how are line features supposed to work in interpolation plugin? Or  
> how can i give structure lines hight values that make sense?
>
> Well, somehow you need to say the plugin what the values to interpolate  
> are. Currently possible is to take from attribute or from z-value (for  
> 2.5D geometry types). Where do you have the height values in your data?
>
>  >So, how are line features supposed to work in interpolation plugin?
>
> The line features are used to modify the triangulation. The  
> triangulation is modified such that no edge crosses a breakline or  
> structure line (constrained Delaunay triangulation). The difference  
> between handling of breaklines and structure lines is that cubic  
> interpolation produces a sharp break rather than a continuous one.
>
> Please note that the interpolation plugin has some numerical problems in  
> the TIN creation that may lead to crashes (especially for data in  
> degrees). There is the option to dump the generated triangle net, so you  
> can see what the plugin does.
>
> Regards,
> Marco
>
> On 23.07.2013 23:44, Bernd Vogelgesang wrote:
>> Hi folks,
>>
>> i'm currently playing around with a set of LIDAR points (200m Grid, but  
>> for free).
>> Trying to "pimp" the model up, i also added some hight points from a  
>> topo-wms and wanted to add break lines and structure lines (streams).
>>
>> Now i'm a bit clueless about the usage of the line features. The  
>> interpolation plugin always asks for a interpolation attribute, which  
>> doesn't make much sense cause those lines don't have them. They choice  
>> of the attribute field seems automatically limited to numerical fields  
>> (which makes sense for the z-values of points).
>>
>> When i just create and pick a numerical field without values, the lines  
>> seem not to have any effect on the interpolation. When i use e.g the  
>> id, it has bizarre effects on the outcome.
>>
>> So, how are line features supposed to work in interpolation plugin? Or  
>> how can i give structure lines hight values that make sense?
>>
>> All tutorials and documentations (e.g.  
>> http://docs.qgis.org/1.8/html/en/docs/user_manual/plugins/plugins_interpolation.html  
>> ) i found on that only cover the unproblematic usage of points and  
>> leave out the lines completely.
>>
>> Thanx
>> Bernd
>>
>
>


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