[GRASSLIST:4496] RE:Two questions: 1. r.param.scale; usage and etc... an error got into my last post!!
larsf at powertech.no
Fri Sep 13 18:07:53 EDT 2002
On Thursday 12 September 2002 08:22, Markus Neteler wrote:
> I think that r.param.scale was updated *after* we wrote that section.
> That means that the default values are different, maybe leading to bad
> results with integer DEM. I will investigate after GRASS conference.
> > (By the way a good book :-)!)
> > 2. Is there a way you can to a "density" analysis a la ArcView
> > SpatialAnalyst; with sites (i e point) data? How?
> The s.kernel is close to be released. It seems to do it's job :-)
> But: could you write in detail what SpatialAnalyst is doing when doing
> "density" analysis?
Markus and others!
Sorry for a little late response to this, but I am doing this list from my
home using my private e-mail adress.
Enclosed you will find a t.xt file with the text from the relevant help
section in ArcView 3.2 with SpAnalyst v 2.0 installed. I think Erik G Miller
is pretty accurate in his response on this. Though it does allow you to
choose betwen simple and kernel density type. And it also calculates the
density based on a circle, the diameter of witch you give as a parameter. In
"The ESRI guide to GIS analysis" vol 1: p 75-85 describe the methodolgy
behind density surfaces in SA.
The s.kernel module sounds to be pretty much what I am looking for.
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Activate a point or multipoint theme.
2 From the Analysis menu, choose Calculate Density.
3 If the view's analysis properties haven't been set to a specific value, then you'll be prompted to set the extent and cell size for the output grid theme. Do so in the Output Grid Specification dialog and press OK. For more information on setting the analysis extent and cell size using this dialog, see Output Grid Specification (Dialog box).
4 In the Calculate Density dialog, choose a field from the Population Field dropdown list to identify the measured quantity to distribute through the output grid theme. A choice of <None> assigns a value of 1 to all points in the input point theme. The value of Population Field for a multipoint feature will be used for each point in that feature.
5 Specify a search radius in the Search Radius type-in box. The radius determines the distance to search for points from each cell in the output grid theme. The radius is reported in the distance units specified for the view. If the view's distance units are not set, then the search radius is reported in map units. By default, the radius value is initially set to the smaller distance of the width and height of the extent of the input point theme (or the extent of the selection) divided by 30.
6 Choose either Simple or Kernel in the Density Type dropdown list.
7 Choose the units that you want the density values to be presented in from the Area Units dropdown list. Press OK.
Calculate Density distributes the measured quantity of an input point theme throughout a landscape to produce a continuous surface.
The two types of density methods are
Simple - Density is calculated for each cell by summing the value found in Population Field for each point found in the Search Radius and dividing by the area of the circle in Area Units.
Kernel - Density is calculated the same way as with the Simple method except the value found in Population Field is distributed out from each point. The result is a smoother looking output.
The output density values will be the occurrences of the measured quantity per specified Area Unit. If the map units of the view are not set, then the Area Units are set to Square Map Units and cannot be changed. If the map units of the view are set, Area Units can be Square Miles, Square Kilometers, Acres, Hectares, Square Yards, Square Inches, Square Meters, Square Centimeters, or Square Millimeters.
Calculate Density works off of the selected set of the active theme. If the active theme does not have a selected set, then all points in theme are used. For more information on selecting features, see Selecting features on a view.
The output grid theme from Calculate Density is automatically named "Density from" followed by the name of the input theme. The grid data set associated with the output theme is written to the project's working directory, with the name "denst" followed by a unique number. Use Properties in the Theme menu to find out which data set is associated with which theme. Use Properties in the Project menu to change the project's working directory. The grid data set associated with the output theme is temporary and will be deleted when the theme is deleted. Use Save Data Set in the Theme menu or save the project to prevent the grid data set from being deleted when the theme is deleted.
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