Latest Contributions by LauraCockrell

‎03-16-2013

Hello- I have a shapefile with polygons of sampled wetlands. I want to separate the shapefile into a series of shapefiles of each sampled wetland. I know I can select each individual feature within the attribute table and use Export Data>Selected features to create each shapefile, but I was hoping there was a tool of some form that would do it (There are 36 features in the original shapefile, so not a ton but enough that I don't want to have to do each individually). Thanks for your help! ~Laura Cockrell

‎03-16-2013

Hello folks- I am using the raster calculator to build the NDVI expression (using Arc10.0) because I want the raw values of the NDVI calculation. However, in order to use the raster calculator and get the raw values, I have to change my original landsat input into float images (I used the Math toolbox>Float tool to convert). The resulting output from the raster calculation of NDVI is a 32 bit float image. The NDVI looks great, I can use classified symbology to break up the color ramp and I really like the display, but I want to work with the pixel values to calculate the mean pixel value for the habitat that I have selected. Obviously with the NDVI being a 32bit float I can't build a raster attribute table. Is there a good way around this? Ultimately, I want to clip out the areas I sampled, then calculate the mean NDVI across each area. Then I want to remove all the negative NDVI values and calculate the mean of the remaining positive NDVI values and see if either of those NDVI's can be correlated with site use. Thank you for your help! ~Laura Cockrell

‎03-14-2013

Eric- Thanks for the advice, I ran the raster calculator to run the NDVI and it maps it out with the values -1 to +1 values. However, I want to get those pixel values, and I don't know how to approach that now! Obviously by changing the original raster to a float to work out the raster calculation, the output NDVI is in float and I can't build an attribute table. Is there another way to find the pixel values of the NDVI equation? Specifically, I ran the NDVI over a large area, and I want to find the NDVI values of the individual habitats within the larger raster. So I guess the mean value of the wetland as a whole is all I really need, maybe there is an easy way to derive mean values of the selected pixels? Thanks! ~Laura

‎03-11-2013

Eric- In the upper left-hand corner of the IAW is the Image Analysis Options. When I open that, the first tab is the NDVI tab. All it says in that box is Red Band: 3 and Infrared Band: 4 Is this an option only on certain licenses? Thank you for your help ~Laura

‎03-09-2013

I am looking at a 2005 landsat image, and I used the IAW to calculate the NDVI... easy-peasy! But everything I have read about the NDVI's says that the resulting calculations should be in a range from -1.0 to +1.0 with negative number signifying vegetation loss and the positive values signifying vegetation gain. I am not seeing this. When I create an attribute table from the NDVI raster, the values are on a 0-256 scale. How do I create the NDVI values for the raster in the -1.0 to +1.0 range?

‎01-14-2012

1. The original raster is a 16-bit, continuous, unsigned integer. 2. When the difference layer is produced on-the-fly in the IAW, the product is displayed as a color map. I would like to change the display to Classified and break it into three classes. ---->In lue of this not working, I clipped out the area I am working with as a subset, and THAT I was able to classify, so it is likely that the original raster difference was too large for the function to work. 3. The difference in raster values of the area is -87 to 136 when I use std-dev stretch on the display and -32768 to 32767 when I use a the 'none' under stretch. Not sure if there is a better way to find out that info. 4. The difference data set is 26.85 mb, 3703 columns and 3801 rows of pixels. The landsat image I am working with represents just over 2.2 million acres of land. As I said above, I clipped the Difference layer with the subset of habitat that I sampled and I was able to break down the classification the way I wanted it. I am just guessing, but it is likely that my laptop was unable to handle the total processing size of the raster and it can handle the size of the data subset much better. ~Laura Cockrell

‎01-10-2012

Hi all, I am using the 'Difference' button to process two rasters using the Processing feature in the Image Analysis Window could use some help with the resulting image. The image produced is displayed using Colormap and I want to produce a classified raster that shows the difference as three classes: Gain (+ 1.1 to positive X value), No change (-1 to 1), and Loss (-1.1 to negative X value). When I click on Show: Classified, I get a 'Compute Histogram' question box that tells me "The Classified renderer requires the data to have a histogram. Do you want to compute the histogram?" I click Yes and get a 'Classify Renderer' information box that tells me "Unique histogram is not available. You can import class breaks from another layer." Then, 'Initialize Classify Renderer' information box pops up with "Failed to initialize classify renderer (possibly too many unique values)". I tried to Build Raster Attribute Table for the difference raster, because I figured that if the attribute table was there to define the values of the pixels, it would be easier for GIS to compute the histogram for the classification. I get back "Error Message 000049: Failed to build attribute table" and am instructed that I should make the raster is a single-band integer dataset. Not really sure what that means... Under raster properties I know my raster is a signed integer raster and has a 16-bit pixel depth. When I try to Calculate Statistics, I get a Python script error, everything freezes and then GIS crashes 😕 I was working on this and somehow got it to work, but cannot seem to duplicate it even though I followed my steps from before. Any advice on working through this issue (or any suggestions on working around it) would be very much appreciated!! ~Laura Cockrell

‎01-07-2012

Thanks Fiona. I am using the Red-to-Green diverging bright color ramp for my NDVIs and I really think it helps display the differences better than a gray scale any day of the week. But like you, I am confined to my advisor's wishes! He just hasn't expressed an opinion on that... yet. I am curious why NDVI automatically displays with a colormap symbology rather than a stretched color ramp in the first place. Perhaps this us just me, but I think the stretched color ramp shows the difference in the vegetation better.

‎12-19-2011

I am using landsat images to build NDVI maps trying to assess wetland plant vigor. I am curious from people who have used NDVI in the past, how do you prefer to manipulate the image for better interpretation. As in: What kind of Contrast is best? What kind of Brightness do you prefer? Do you apply a gamma stretch (and if so, what)? When is it better to apply stretch as a std-dev rather than a percent clip? Any useful tips for reading the NDVI is most appreciated!! I am currently using the Image analysis window in Arc10. Thanks! ~Laura Cockrell

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Date Last Visited	‎11-11-2020 02:24 AM

My Ideas

Latest Contributions by LauraCockrell

Separate attributes into unique shapefiles

Have to convert raster... but need to build attribute table!

Re: Interpreting NDVI results

Re: Interpreting NDVI results

Interpreting NDVI results

Re: Processing raster difference in Image Analysis

Processing raster difference in Image Analysis

Re: NDVI Image preference

NDVI Image preference