Geoprocessing : Spatial Analysis

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dmhoneycutt — Wed, 25 Mar 2009 23:47:00 GMT

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New at 9.3 - Spatial Analyst improvements

dmhoneycutt — Thu, 02 Oct 2008 23:08:00 GMT

The Spatial Analyst at 9.3 has several new capabilities, improvements, and fixes. This post highlights some of the important changes; for a full description, see the What's New pdf document (Once open, in the Extensions section click on Spatial Analyst. This will take you to the right location in the document.)

New Functionality

Zonal Histogram - Helps you to investigate the distribution of cells from one dataset within classes of another. Example applications include rainfall within watersheds or crime distribution by police beat.

Contour with Barriers - This tool allows you to get better results when trying to contour discontinuous surfaces. In addition to its superior handling of barriers, Contour with Barriers creates smoother contour lines, generate much larger output datasets (no 2.14 GB shapefile size limit), and optionally to attribute primary and index contour lines.

The Snap Raster environment has moved from being part of the Extent environment to being its own separate environment setting. This gives you flexibility to set the snap raster dynamically from scripting or ModelBuilder.

Usability Improvements

The Con tool now allows the input conditional raster to be a floating point raster.

The Create Constant Raster, Create Normal Raster, Create Random Raster, Extract by Rectangle, and Topo to Raster tools now allow you to specify the extent of the output raster to be that of an existing dataset. The main benefit of this is the extent can be set dynamically from scripting or ModelBuilder. While you can still enter the extent values manually (but who wants to type in 370803.47263702 3978856.7539477 443868.87614745 4032133.0258861 ?), being able to specify a dataset is both easier and helps ensure the proper precision is maintained .

All Spatial Analyst tools have new Geoprocessing tool errors and warnings and show execution percent complete using the new progress bar.

Performance and Data Improvements

Combine and Cut/Fill are significantly faster (up to 40 times).

Viewshed now handles much larger inputs (no more MSEEK fatal errors!).

IDW now works with many more points (up to 50 million).

Combine now works on millions of combinations.

Understanding Geostatistical Analyst Layers

dmhoneycutt — Mon, 29 Sep 2008 19:14:00 GMT

The Geostatistical Analyst (GA for short) uses sample points taken at different locations in a landscape and creates (interpolates) a continuous surface. The sample points are measurements of some phenomenon, such as radiation leaking from a nuclear power plant, an oil spill, or elevation heights. Geostatistical Analyst derives a surface using the values from the measured locations to predict values for each location in the landscape.

The results of an analysis using GA are saved to a geostatistical layer (GA layer for short), illustrated below. A GA layer is like any ArcMap layer; you add, remove, rename, and alter its symbology in countless ways.

GA layers store the source data (typically point features) and the parameters used to interpolate values, but do not store the full result of the interpolation. When a GA layer is drawn in ArcMap, a coarse grid is placed over the whole extent of the layer and values are interpolated on-the-fly for each cell in the grid. These interpolated values are used to create the filled isolines displayed by the layer. These isolines are only produced for 10 classes that fall between the min and max values of the source data. The resolution of the grid is optimized so as to speed up the drawing of the layer; using a finer grid would cripple drawing speed. (Note: when deemed necessary, the drawing algorithm uses a finer resolution grid to interpolate values in complex areas.)

The fact that a GA layer interpolates values on-the-fly is sometimes a source of confusion. For example, when a GA layer is exported to a grid using the GA Layer to Grid tool, interpolation is performed for each cell of the output grid. If your output grid has 1000 rows and 1000 columns, one million calculations are performed. (In contrast, the grid used to draw a GA layer may be 100 rows and 100 columns). Fine resolution output grids may require several billion calculations. Obviously, the performance of the GA Layer to Grid tool depends on the resolution of the output grid.

There is another effect of exporting the layer to a grid — the output cell values, and therefore the min and max values of the output grid, can be affected by the grid's cell size. The min and max values of the grid are almost always different from the min and max values of the input data. For example, the illustration below shows a GA layer with its min and max values and the exported grid with its min and max values. Note both the visual and statistical difference between the min and max values of the two displays.

Exact interpolators, such as Inverse Distance Weighting, can produce an output grid that has a max value that is less than the input data's max value, as illustrated above. This happens when the input point containing the max value does not fall in the exact center of an output grid cell. In such cases, the max value is weighted according to the distance from the cell center as well as nearby points.

Suggested readings

Understanding Geostatistical Analysis

What is a Geostatistical Layer?

Knowledge Base Articles

Geostatistical Analyst Forum

Thank you so much from the Geoprocessing and Analysis Team

bbicking1 — Thu, 21 Aug 2008 14:49:00 GMT

...to Everyone who visited us at the Spatial Analysis Island and attended our Tech Workshops and Island Demos. We do hope you recovered from San Diego and are excited about the new things you learned, the answers you got to your questions, and together all will make your work much easier and more fun.

As always, we also learned a lot from you, about what you like, or don’t like, or want us to do in the future. This is a very special time for us to live vicariously through the great work you have done on top of our work, and understand how to better support you.

Since you’re here, you heard about the just launched Resource Centers, the new gateway to all ESRI online resources. Here’s the link to ours http://resources.esri.com/Geoprocessing/ . Add it to your Favorites and visit the site often, especially the Geoprocessing Blog and the Model and Scripts Gallery [both under Community]. The Model and Script Gallery on the Geoprocessing Resource Center is a bit different from the other resource centers, in that it's not just us posting things, we're inviting you to post your work as well. Whether that be generic utilities or complete workflows for a specific task, if you think it can be useful for someone else, feel free to share it. I talked with several of the more prominent ArcScripts contributors from the past, who are pretty excited about having a more interactive place to post their work and communicate with their users. We have guidelines for submission, which include documentation of your model and script. When putting the documentation together, make sure you include any use limitations/restrictions – such as if it relies on an ArcInfo or extension licensed tool.

Regarding writing scripts, we saw noticeable increase in the number and depth of Python questions at the island this year. Actually, there were a lot of great questions and interesting problems to solve at the island, that have us pretty excited about some things we’ll be doing in future releases that will create a richer Python experience in ArcGIS for you (more on that in the future).

On the other end of the spectrum, we met a lot of new people at the Spatial Analysis Island this year who were attending their first International User Conference, and some who were quite new to the software. For many of them, the team enjoyed demoing ModelBuilder and showing them how to automate their daily routine. It was especially rewarding for those of us who had the chance to introduce many of you to ModelBuilder and hear you say: “Wow, ... I’m gonna use that every day.”

From the more experienced ModelBuilder users, there were quite a few posters with some very sophisticated models. We’ll be reprinting a few of these to decorate the halls around our new offices. Thanks for the nice work.

From the analytical side of the conference, all the interest seemed to be on statistics. Our sessions on spatial statistics, regression, and statistical analysis with Spatial Analyst were generally packed, and lots of in depth discussions at the island on applications of these tools as well as Geostatistical Analyst, and integration with 3^rd party statistical packages. We’ll be posting some samples to the Model and Script Gallery in the near future and working on some webcasts to share more about current capabilities and applications.

We hope you got as much out of the conference as we did, and look forward to interacting with you via the Resource Center until we meet again next year.

Stay tuned…

The Geoprocessing Team