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# Tag Info

37

Both forms rely on Toblers first law of geography: things that are close are more related than things that are further apart. IDW is the simpler of the two techniques. It involves using known z values and weights determined as a function of distances between the unknown and known points. As such in IDW points that are far away have far less influence than ...

9

It is partially explained here http://www.gistutor.com/quantum-gis/20-intermediate-quantum-gis-tutorials/51-inverse-distance-weighting-idw-interpolation-using-qgis.html by first showing examples of using coefficient values of 1 and 3, and then As you can see, a larger coefficient means it takes a larger distance for the values of the surface to become ...

6

Noise is much more complex than a simple IDW interpolation. Sound propagation depends on many factors and distance is just one of them. Air density, temperature, humidity, terrain, wind direction and ground attenuation should all play their part in even the simplest of models. In addition to these simple factors there are issues relating to tonality of ...

5

In general, workflows that you would like to automate I would recommend first doing it manually. Once you have that logic understood (what tools to use when), then yoiu could create a model/python script. For this case here would be the general model workflow (assuming you are using ArcGIS): Use Make XY Event Layer tool to create the GIS layer Use Add ...

5

The short answer is: no, but it might be the best you can do without additional layers. The problem is that humans do not settle evenly. If all you have is populations of cities then IDW might be your best approximation, because population density does decay the further you go from the center of a city (as IDW models). However, population does not decay ...

4

In case you want to make it go faster by reducing duplicated computations: def CalcIDWvalue(unknowncell, knowncells): weighted_values_sum = 0.0 sum_of_weights = 0.0 neg_half_sens = -sensitivity/2.0 for knowncell in knowncells: weight = ((unknowncell.x-knowncell.x)**2 + (unknowncell.y-knowncell.y)**2)**neg_half_sens ...

4

Awesome, well I found out which parts of the code were wrong and got it to work. I ended up removing two segments in the code that only did some action when the unknowncell and the knowncell were different from each other, which is pointless to test for since that should never be the case. Although obviously it must have had some effect since it managed to ...

4

This blog has some explanations, here is a brief excerpt from one of the answers: When you use a barrier, every IDW calculation--that's one per output grid cell--has to involve a check against the barrier file for every possible neighbor. The more features there are (you have 1088) and the more vertices they have (you have 23,938), the longer it ...

4

The original SAGA IDW method does have a parameter for minimum number of points. But this parameter is not used in the QGIS version of the SAGA method. As an alternative you could create a layer that contains only the points that have the minimum number of points within the search radius. Have a look at this answer for the code to make such a selection. ...

4

You can see distance-based interpolation formula in following link: http://www.gitta.info/ContiSpatVar/en/html/Interpolatio_learningObject2.xhtml where p is distance weighting exponent and it probably was the parameter that is different between these 2 SAGA versions. So, it's relatively easy to code this algorithm in PyQGIS. As follow, it has inside ...

4

Maybe I should begin by stating that "the summit issue is reflecting how IDW models the surface, rather than weighting". IDW sees the predicted surface as an averaging model, while Spline tries to minimize abrupt change to make 'smooth rubber sheet' and Kriging tries to minimize errors. (I hope this makes my point clear). Let me focus on the difference ...

4

The bulls eye effect describes concentric areas of the same value around known data points. It's simply an unfortunate artifact of IDW interpolation. The effect gets worse the more isolated your data points are. IDW suffers from this problem more than other interpolation methods (e.g., Kriging), but to a large extent nearly any interpolation method will ...

3

IDW interpolation will not be good because it will not calculate the distance along the polyline. IDW will interpolate from the blue point values and give more weight to those blue points that are closest. For example if your situation was like the one below the red points between 200 and 300 might well receive values greater than 300 because of the ...

3

Presuming your data are points with precipitation as a column, then: load them into QGIS From the menus: Raster/Analysis/Grid (interpolation) Choose your points as input Set the Z field to your precipitation column Choose an output file Select 'inverse distance...' as your algorithm Set your other parameters as required Press OK

3

I believe that you can specify an IDW with a radial basis function in the geosptdb library using the "rbfST" funciton. The package is intended for spatial-temporal IDW but should run with a single temporal dimension. This is at least a good place to start and you may be able to modify the "rbfST" function to suit your specific needs.

3

Blockquote Can my process/decision for interpolation be adequately justified for a paper, or should I be doing something else? Well, +1 for this one. Bear with me, I am not a (geo-)statistician at all, but I am always a bit stumped when I see people trying to interpolate datasets that simply aren't suitable for interpolation, even in the face of ...

3

You have a classic zero inflation problem and this data is just not suitable for interpolation statistics. You may want to try a regression approach using a zero inflated model (ZIP), where the zeros are modeled independently as a binomial process. Commonly, the non-zero model is a Poisson regression but, that is not set in stone and could be any ...

3

By default, the environment is set to the same coordinate system as the input data (not the map), but maybe this is overwritten in your case (main menu > geoprocessing > environment). From the IDW tool, press the "environment" button, go to "Output coordinates" and manually select you UTM projection. The unit should then be in meters. As a remark, you can ...

3

There is indeed a nice overview page available here: https://grasswiki.osgeo.org/wiki/Contour_lines_to_DEM The Wiki page demonstrates and compares a number of different methods of converting vector contour lines into raster DEM surfaces. This includes a series of graphical examples of the output to be expected.

3

In order to evaluate the accuracy of your interpolated surface you either need: additional samples or to remove some random samples prior to interpolation These are used to cross-validate your result. On your created IDW layer you right-click and choose Validation/Prediction. This link actually describes the process: https://desktop.arcgis.com/de/...

3

Yes, you can call this from a Python script. But you don't directly call the low-level C API. First, take a look at the GDAL Grid Tutorial for background info. From the Python library, the relevant function is gdal.Grid(destName, srcDS, **kwargs). You can see some examples of how it's used in test_gdal_grid_lib.py (from the test suite). Or a made-up example:...

3

Package sf also provides sf::st_make_grid library(sf) library(gstat) data <- as.data.frame(data_to_analise) colnames(data)[1:3] <- c("longitude", "latitude", "z") st.sf <- st_as_sf(x = data, coords = c("longitude", "latitude"), crs=NA) colnames(st.sf)[1] <- "z" vgm1 <- gstat::variogram(z~1, st.sf) fit1 <- gstat::fit.variogram(vgm1, ...

3

The output setting reveals it will be a 1 pixel raster. You will likely want to change that to have more! The points seems to be in 4326, so the unit is "degree". The output pixel size is 0.1 degree. You can see from the input extent that the longitude spans between -53.577 and -53.553, so you would need a size of 0.0001 degrees (or even less!) to start ...

2

For my masters thesis I ran a comparison of interpolation methods for interpolating elevation maps from LiDAR clouds, and in almost all scenarios thin plate spline interpolation was the most accurate method. IDW is good in some situations but it's quite primitive. If you're dealing with continuous data types (as opposed to categorical data types), which I ...

2

I wouldn't waste my time trying to do this with ModelBuilder. It'll drive you crazy. However, you could do this trivially with Python using a pair of nested loops. Pseudocode: list_of_fields = ['field1', 'field2', ...] for row in table: for field in list_of_fields: perform IDW on row with value of field

2

You may use Iterate Field Values on your feature class for this task. Iterates over each value in a field. http://resources.arcgis.com/en/help/main/10.2/index.html#//00400000001n000000

2

I would recommend using Zonal Statistics as Table rather than just Zonal Statistics -- since your end goal is a table (in Excel), it is a more direct workflow. The statistics are the same, but the output is a table. I am not entirely sure whether Excel is able to open .dbf files (one of the available table formats) directly; if not, you can open it within ...

2

The default value (as you can see from the description) is somewhat arbitrary. The shorter of the x or y axis (extent) divided by 250 has no real logic as best I can tell. In Python you should set this as a user defined parameter and set the default as what works best for your area / analysis.

2

I find the textbook that gives the best definition and discussion is: Principles of Geographical Information Systems (Spatial Information Systems) Paperback – April 9, 1998 by Peter A. Burrough Rachael A. McDonnell I seem to remember a trial dataset of flooding on the Netherlands as well to play with.

2

As far as I know, IDW is implemented as an exact interpolator in ArcGIS Geostatistical Analyst, as reflected in the Prediction Map results. This issue occurs only when you convert to raster, as you point out, and is intended. The raster resolution is finite, and the values of the cells are either taken as a value in their center or as a mean over their area ...

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