semivariogram + coordinate units
Thanks for the advice. I found this comment on a web page (from a geostats class): "...If the data coordinates were in different units then we would need to standardize these coordinates (otherwise km in the horizontal and m in the vertical produce very flatten out grids)...", so I'm wondering that if by using "relative" locations (say 0 to 1) instead of the absolute ones (cm or m) can be utilized as an alternative for the analysis. The point is that by transforming everything to meters I'm having some problems to fit the models and I guess is due to the reduced range of the x-axis (0-0.02 m) compare to the y-axis (0-20 m). BTW: the grid represents a set of data points measured within a pole, so that's the reason of the differences in scale (x is from the center of the pole to the outer part of the pole) and y from the bottom to the top. PM On Fri, Apr 11, 2008 at 9:22 AM, Steve Friedman
<friedman.steve at gmail.com> wrote:
Pedro, You absolutely need to have the coordinates in the same units of measurement. The geostatistics model is based on the premise of location, distance between samples, and the variance between locations. The bigger concern that I can think of is the "change in support" For example lets hypothesize that the variance changes much faster in the x direction which in your case is measured in cm. So far that seems ok, but how can you tell whether the variance is changing at the same spatial lag in the y direction which is measured (and perhaps sampled) 10 more crudely? Changing the coordinates to the same units of measurement is a transformation that may or may not mean much if your sampling scheme is problematic. Good luck Steve On Thu, Apr 10, 2008 at 9:19 PM, Pedro Mardones <mardones.p at gmail.com> wrote:
Dear list members; I'm a newbie in geostatistics so this question may have an obvious answer that I'm not aware of (sorry about this). I've a grid of points in which the x-axis is measured in cm (from 1 to 20) and the y-axis in meters (from 1 to 20). So basically I have 20 x 20 points to work with. Here is where I'm kind of confused. Do I need to transform the coordinates to the same units, say cm or meters, before obtaining an empirical semivariogram? What could be the effect of using the coordinates in the given units (ie without transforming them to the same units) on the analysis? Thanks for any hint PM
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