[Qgis-developer] Processing params always n meters?

[Raymond Nijssen]

Hi Paolo,

I've been thinking of this as well and I would love it! I hate ending up 
with a huge circle on my screen after performing a quick "look how easy 
qgis processing is!" 500 degrees buffer on my data. :)

The problem is that you cannot convert degrees to meters so easily since 
they depend on the location on earth. I'm sure you know.

So even for smaller areas, for example my house in the Netherlands, 
there would already be a 1 : 1.5  difference between lat:lon. Pretty 
hard to tell the buffer function.

On smaller scales, for example a country map of Chile, there would also 
be big differences in the distances on the north and south sides of the 
country.

So I would say, don't do it! I'm afraid this is the reason for being 
stuck with all these map projections in geography.

Still, qgis could improve on this I think:

1. Automatically reproject the data to the appropriate CRS (?), perform 
the analysis, and reproject back. UTM could be used for it, hoping all 
data is in a single zone.

2. Showing the CRS units in the analysis dialog, maybe even in 
red/italic/bold when these are degrees.

I'd vote for option 2 here, since I like to know what happened to my 
data. Buffers performed in different (overlapping) CRS's won't be exact 
the same.

Regards,
Raymond

Btw, here is a nice example story about this:
https://miamigeographic.com/2014/07/22/into-perspective-miami-world-city-distances/




On 05-03-17 08:15, Paolo Cavallini wrote:
> Hi all,
> when analyses are done on degrees CRS, often parameters are difficult to
> use (e.g. buffer width in degrees is not easy to calculate). The usual
> strategy is to convert layers first to a metric CRS.
> Would it make sense to always input params as meters, and reproject the
> parameters as appropriate? This would be far easier for the user.
> All the best.
>