Showing posts with label resample. Show all posts
Showing posts with label resample. Show all posts
Wednesday, August 9, 2017
Tuesday, October 25, 2016
Comments on Prepare Rasters for Maxent versus Find ArcGIS Rasters and Project to Template in MGET
On my page for the Prepare Rasters for Maxent Tool for ArcGIS I made a comment about its application versus another tool called Find ArcGIS Rasters and Project to Template in the Marine Geospatial Ecology Toolbox. To access that page click HERE . Here is the text of that comment:
The Marine Geospatial Ecology Tools for ArcGIS (http://mgel.env.duke.edu/mget) is an excellent toolbox that includes much of the functionality of the Prepare Rasters for Maxent toolbox. I highly recommend checking it out. In particular, check out the Find ArcGIS Rasters and Project to Template tool which will batch project and clip all rasters to a template. One primary difference between that tool and Prepare Rasters for Maxent is that Prepare Rasters for Maxent has built-in steps to resample and fill in missing data. If you know that you have gaps in your data then Prepare Rasters for Maxent may be your best bet. If you don't think that you have gaps or they are minor then consider Find ArcGIS Rasters and Project to Template as a speedier alternative. You can create a template raster by multiplying all of your rasters together in the Raster Calculator. By default this will take the intersection of all of the rasters resulting in a template that is smaller than any one of the inputs.
The Marine Geospatial Ecology Tools for ArcGIS (http://mgel.env.duke.edu/mget) is an excellent toolbox that includes much of the functionality of the Prepare Rasters for Maxent toolbox. I highly recommend checking it out. In particular, check out the Find ArcGIS Rasters and Project to Template tool which will batch project and clip all rasters to a template. One primary difference between that tool and Prepare Rasters for Maxent is that Prepare Rasters for Maxent has built-in steps to resample and fill in missing data. If you know that you have gaps in your data then Prepare Rasters for Maxent may be your best bet. If you don't think that you have gaps or they are minor then consider Find ArcGIS Rasters and Project to Template as a speedier alternative. You can create a template raster by multiplying all of your rasters together in the Raster Calculator. By default this will take the intersection of all of the rasters resulting in a template that is smaller than any one of the inputs.
Thursday, May 28, 2015
Updates to the Prepare Rasters for Maxent Tool - upscaling rasters
Maxent software (http://www.cs.princeton.edu/~schapire/maxent) is frequently used for presence-only species distribution modeling. Maxent requires, however, that input ASCII raster files be aligned with one another and have the same spatial extent. This tool pre-processes raster data in preparation for Maxent modeling to ensure that all rasters have the same extent, same cell size, and aren't missing data. You can download the tool HERE.
There is a new tool that can be used to upscale data. Upscaling is useful when simple resampling is insufficient. For example, if you have a 30 meter classified vegetation map simply using the majority resample to resample to 1000 meter cell size would be insufficient because it would just tell you about the majority vegetation type present. Better would be to count the number of pixels of the vegetation type of interest within the 1000 meter cells and calculate the proportion of that vegetation type. This also solves the problem of having to treat the data as categorical in Maxent. Vegetation type would have to be treated as a categorical variable whereas proportion of X vegetation type is a continuous variable.
This example illustrates the idea of upscaling.
In the image above we have a forest/non-forest GIS layer overlaid on a coarse-resolution grid. The need for the coarse resolution grid is due to uncertainty in the location of the occurrence points and the resolution of other datasets that will be used in Maxent (e.g. the MODIS satellite products have a cell size of 250 - 1000 meters depending upon the product). The image below shows tree cover at the 1000 meter scale derived from running the Upscale Rasters for Maxent Tool. An important pre-processing step is deciding which vegetation types should be included in the model and which should be omitted. Included vegetation types should be reclassified as 1 prior to running the tool and omitted vegetation types should be reclassified as 0.
There is a new tool that can be used to upscale data. Upscaling is useful when simple resampling is insufficient. For example, if you have a 30 meter classified vegetation map simply using the majority resample to resample to 1000 meter cell size would be insufficient because it would just tell you about the majority vegetation type present. Better would be to count the number of pixels of the vegetation type of interest within the 1000 meter cells and calculate the proportion of that vegetation type. This also solves the problem of having to treat the data as categorical in Maxent. Vegetation type would have to be treated as a categorical variable whereas proportion of X vegetation type is a continuous variable.
This example illustrates the idea of upscaling.
In the image above we have a forest/non-forest GIS layer overlaid on a coarse-resolution grid. The need for the coarse resolution grid is due to uncertainty in the location of the occurrence points and the resolution of other datasets that will be used in Maxent (e.g. the MODIS satellite products have a cell size of 250 - 1000 meters depending upon the product). The image below shows tree cover at the 1000 meter scale derived from running the Upscale Rasters for Maxent Tool. An important pre-processing step is deciding which vegetation types should be included in the model and which should be omitted. Included vegetation types should be reclassified as 1 prior to running the tool and omitted vegetation types should be reclassified as 0.
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