(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 <[email protected]> Date: Tue, Aug 12, 2014 at 4:07 PM Subject: Re: [Qgis-user] Question regarding point to grid analysis To: Carlos Cerdán <[email protected]> On Tue, Aug 12, 2014 at 3:31 PM, Carlos Cerdán <[email protected]> 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 -- [email protected] -- [email protected]
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