38,081 reputation
992165
bio website quantdec.com
location Northeastern US
age 14
visits member for 4 years
seen 2 hours ago

Consultant (environmental and spatial stats a specialty), expert witness, and teacher. I can be reached through (outdated but still valid) links posted on my web site.

Twitter: @WilliamAHuber // ASA-P website: http://amstatphilly.org/


Why waste time learning, when ignorance is instantaneous?

--T(iger) Hobbes.

For any complex problem there is a simple solution. And it's always wrong.

--[Mis?]attributed to H.L. Mencken by Dava Sobel, Longitude.


2h
reviewed Close Is there an editing function in qgis cloud?
2h
reviewed Leave Open Hardware requirements for a modern GIS workstation
2h
reviewed No Action Needed How can you create a depth raster from a DTM and a polygon in QGIS?
2h
reviewed No Action Needed OpenLayers 3: “goog is not defined”
15h
comment Get the specified feature by direction
Could you clarify the basis on which the target is selected? Is it perhaps the one line out of the two whose bearing is closest to the specified direction?
23h
comment How to create least-cost path between two polygons with GRASS?
Thank you for the clarification. Do you know how to find a costdistance raster for a set of origination points? If so, you're practically done, because a zonal summary of that (using the other polygon as the zone) will finish the job.
1d
answered Geodetic lat/lon to ECEF cartesian: Direction to apply geoid undulation from ellipsoid surface?
1d
comment How to calculate a raster area with Grass - help for query by coordinates
Please explain what you mean by "find an identity" in a DEM. (This sounds like a literal translation into English of a non-English phrase, but it does not have any meaning in English without some further explanation.)
1d
comment How to create least-cost path between two polygons with GRASS?
One can imagine many different interpretations of your question: is the "cumulative cost of moving between two polygons" equal to (a) the smallest cost to get from any point in one polygon (say polygon "A") to any point in the other (polygon "B"), or (b) the average cost to get from points in A to points in B, or (c) the average smallest cost to get from points in A to some point in B, or (d) the same as (c) but reversing the roles of A and B or (e) the maximum over A of the smallest costs to get from points in A to B, ... etc, etc. Please edit this question so it states what you need.
Aug
29
comment Arcpy Cell Statistics - maximum number of input rasters
+1 For many reasons, sticking thousands of files in one folder is rarely a good idea. A better organization would have the files arranged into a hierarchy of folders, each containing far fewer datasets. That would practically force one to implement this (better) solution in the first place.
Aug
28
comment Better-matching bounding box by using collection
I'm pretty sure you've seen the packing polygons question. The approach--if not the specific method--I described there will work here, provided you make your question a little more precise and quantitative. You need to stipulate how the program should determine the quality of any approximation. I would guess that fewer rectangles are better and that a smaller error (difference in areas) is better, but what is still missing is some indication of how it should trade off between the two objectives. Can you clarify that issue?
Aug
28
comment How to calculate 90% confidence interval in a raster stack
Since these seem to be a time series of rasters, you ought to consider assessing the possibility of a significant serial correlation coefficient first, because that would (strongly) affect the confidence intervals.
Aug
28
comment Raster diff: how to check if images have identical values?
@Remco The algorithm underlying numpy.unique is going to be more computationally expensive (both in terms of time and space) than most other ways to check that the difference is a constant. When confronted with a difference between two very large floating point rasters that exhibit many differences (such as comparing an original to a lossy compressed version) it would likely bog down forever or fail completely.
Aug
28
comment Raster diff: how to check if images have identical values?
I believe ArcGIS has a built-in capability to build a VAT. There does not seem to be any indication in your answer that you are contemplating a purely Python solution, so you might want to clarify that point.
Aug
28
comment What is an efficient Algorithm to draw a straight line from points?
@Devdatta Sorry for being so terse: I was referring to a lexicographic sort on the x and y coordinates. (Such sorting is often a first step in many algorithms of computational geometry.) It is accomplished simply by sorting on one of the coordinates and then using the second coordinate to resolve any ties. This sort is built in to many programming environments. Finding the endpoints needs only O(n*log(n)) time and O(n) space (or O(n) time with a clever algorithm). It works well here because it automatically handles points that are lined up perfectly vertically or perfectly horizontally.
Aug
27
comment Raster diff: how to check if images have identical values?
It seems likely RAM would not be an issue provided you stick with ArcGIS native operations. It's pretty good with RAM usage when processing grids: internally it can do the processing row-by-row, by groups of rows, and by rectangular windows. Local operations like subtracting one grid from another can operate essentially at the speed of input and output, requiring only one (relatively tiny) buffer for each input dataset. Constructing an attribute table requires an additional hash table--which would be minuscule when only one or two values show up, but could be enormous for arbitrary grids.
Aug
27
comment Raster diff: how to check if images have identical values?
Subtraction seems like a good way to conduct a comparison. However, I believe the histogram would not be very useful in detecting problems with NoData values. Suppose, for instance, that the compression procedure eliminated a one-pixel border around the grid (this can happen!) but otherwise was accurate: all differences would still be zero. Also, did you notice that the OP needs to do this with 7000 raster data sets? I'm not sure he would relish examining 7000 plots.
Aug
27
comment Raster diff: how to check if images have identical values?
One huge way to fool these statistics would be to permute the cell contents (which can happen, and does, when image dimensions are not quite right). On very large rasters neither the SD nor the mean would reliably detect a few small changes scattered about (especially if a few pixels were just dropped). Conceivably they would not detect a wholesale resampling of the grid, either, provided cubic convolution were used (which is intended to preserve the mean and SD). It would seem prudent instead to compare the SD of the difference of the grids to zero.
Aug
27
comment Raster diff: how to check if images have identical values?
Would this work with 32-bit floats? Would building and comparing two tables actually be any faster (or easier) than examining the values of the difference of the two rasters (which in principle should be just zero and NoData)?
Aug
27
comment Is the Azimuth on the equator equal to the one not on the equator?
@radouxju An azimuth can be uniquely defined at all points on any surface of revolution except its poles, if any: that includes the sphere and all ellipsoids. It is given by the oriented angle between a direction and the direction of the meridian at that point. I believe this question is asking for the bearings ("azimuths") of geodesics on ellipsoids. More about this appears after the edit to my answer at gis.stackexchange.com/a/6824. As far as I can tell, that answer fully addresses this question; I do not know why this question was reopened.