[Qgis-user] Fwd: Question regarding point to grid analysis

[Mark Coletti]

(Just now noticed that by default replies *do not* go to the mailing list.
 Oops.  In any case, Carlos insists that I try the point sampling plugin,
which I'm doing now.  Given that I'm dealing with 5 million points, this
could take a while.)

---------- Forwarded message ----------
From: Mark Coletti <mcoletti at gmail.com>
Date: Tue, Aug 12, 2014 at 4:07 PM
Subject: Re: [Qgis-user] Question regarding point to grid analysis
To: Carlos Cerdán <sig.upagu at gmail.com>


On Tue, Aug 12, 2014 at 3:31 PM, Carlos Cerdán <sig.upagu at gmail.com> wrote:

> Hi Mark:
>
> I think that "point sampling tool" plugin is the tool what you need, With
> it, you take the corresponding raster value into the points attribute
> table, so in a new column you can calculate difference between point value
> and sampled value and make a statistic analysis of that difference.
>

There are a number of problems with this.  First, you say "take the
corresponding raster value into the points attribute table," which implies
going from raster to vector when I would be doing the exact inverse.  I.e.,
I **start with** vector data to then derive raster data.  Second,
"sampling" has the connotation of taking a subset of values from a larger
set when I want to use **all the values** and not a subset.

I'll make my intent more explicit:


   1. convert point layer P_1 to raster layer R_1 using P_1's attribute M_1
   2. convert point layer P_2 to raster layer R_2 using P_2's attribute M_2
   3. perform measures of similarity between R_1 and R_2

Moreover these constraints must be satisfied:


   - R_1 and R_2 have identical minimum bounding rectangles; i.e., they
   cover the same area
   - R_1 and R_2 have the same cell sizes

The first challenge is noting that there exist many ways of converting a
point layer to a corresponding raster.  E.g., heatmaps, MMQGIS gridify, and
several Processing "Vector -> Raster" algorithms.  The second challenge is
that once I decide on a means of creating a grid layer from vector that
those constraints are satisfied.

I know qgis is capable of doing this task, I just need some pointers on how
to proceed.

Cheers!

Mark
-- 
mcoletti at gmail.com




-- 
mcoletti at gmail.com
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