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Thank Dan! Indeed I had not found some the links you suggested. I'll do some more reading and see if all my questions are answered ;). And you are right about the low sample size for some of my ellipses. Thankfully, most have hundreds if not thousand of points.
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10012015
09:44 AM

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53

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Hi, I am using the Directional Distribution (Standard Deviational Ellipse) tool under the Spatial Statistics toolbox and Measuring Geographic Distribution toolset for an analysis I wish to publish. The results of running the tool looks great but I want to make sure I understand how they were created. The ArcGIS help website mentions “When the underlying spatial pattern of features is concentrated in the center with fewer features toward the periphery (a spatial normal distribution) a one standard deviation ellipse polygon will cover approximately 68 percent of the features; two standard deviations will contain approximately 95 percent of the features”. The key word in that sentence is “ When […]”. How do I know whether each ellipse follows a spatial normal distribution? If my spatial distribution is not normal, how are the ellipses created? Does the tool ignores the two standard deviations requested option? I have less than a handful of ellipses that were created using less than 5 points. Regardless of the low sample size, those ellipse seem to have a buffer between the location of the points and the boundary of the ellipse with the ellipse being larger. How was the size of the ellipse determined? I can provide a figure if needed. Thank you for all your help,
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09302015
11:40 AM

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2515

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Thank you for the article about how the points are randomly selected. It was very useful. Following on 1). The number of points value I mentioned in the tool was always higher than could fit in my study area (about 3050 points could fit) but lower than the number of points in the constraining feature layer (5300 points). The article you mentioned I believe answered my question in that "This process is repeated until the specified number of points is reached". In this case, since the number of point values cannot be reached but no more points can be added, the process would just stop and thus the maximum number of points that can fit would indeed be selected (I think). Running the tool does result in a warning ("The specified number of points could not be created in all cases due to restrictions from the minimum allowed distance") but the tool still runs. Following on 2). Found the solution! I changed the projection of the input layer to UTMs which uses meters (instead of the decimal degree it used to be). Et voila! The minimum distance in meter is now respected. As a side thought, this is the second time within the same project that changing the projection of the input layer results in the tool working properly (had problems going from UTMs to MGRS in the Convert Coordinate Notation tool). If specific projections are needed for the functioning of a tool, one would think the information would be mentioned in the instructions (or help menu) or the tool fixed? Anyway, things are working now. Thanks!
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02102014
08:02 AM

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39

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I am using the tool Create Random Points to create points with a mininum distance of 150m within an area. The constraining feature is a point layers consisting of points every 10 meters (center point of a MGRS grid). I have a question and have been experiencing a bug. 1) It seem mandatory to mention a number of points value. My goal is to create (select really) the maximum number of points that can fit within the area respecting the minimum distance. Not knowing what that number would be, I have put a number of points value exceeding what I think can fit. Is the tool going to fit in as many points as possible? 2) I have mentioned a minimum distance of 150m under the linear unit option. The points created are however sometimes closer than 150 m (110m, 130m, etc.). Any reason why that would be? Thank you for your help.
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02072014
10:59 AM

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747

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Doing some brainstorming to encourage discussion . . . 1) Should the raster be converted to a a point layer? Although the points would all be at equal distances, could the heterogeneity of the values (i.e. bird abundance) still be quantified? 2) Whether using raster or vector tools: how is spatial heterogeneity defined using GIS tools? Are some more reflective of ecological processes? 3) Within a specific polygon size (eg watershed scale), can patches of nonsignificant differences in values (i.e. similar bird abundance) be created? If so, number, location and size of patches of similar bird abundance within each polygon (i.e. watershed) would be interesting. 4) What tool gives average distance of significant difference between points (or cells) within a polygon? If patches (i.e. polygons) are created from the point/cells, is there a tool that defines the amount of edges (i.e. there would be a lot more edges in a highly heterogeneous landscape). 5) . . .
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02192013
08:50 AM

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33

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I have a raster layer representing bird abundance and would like to quantify the spatial heterogeneity of their abundance at 3 scales. Each scales is represented by a vector layer with a particular size polygons. Any recommendations on which (raster) tool could help me? Thank you!
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02152013
02:05 PM

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