Image data format conversion
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Image data format conversion
By edi151 in ERDAS Software Forum
Hello,
Does ayone know a way in ERDAS to convert 16bit data format images (processed Landsat images) to 8 bit unsigned data format images. I appreciate that the originally released format for the imagery was 8 bit unsigned.
Using Radiometric/Rescale or changing the output data type under Spectral/Restack results in most of the output image having pixel values of 255. In other words it hasn't rescaled from 0-32,768 to 0-255.
Maybe Rescale is not working on my installation? I'm using ERDAS Imagine 2011 with the latest updates.
Thank you to all the good folks who kindly help in this forum. Any assistance would be very much appreciated.
Sincerely,
Edi
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Re: Image data format conversion
By Donn Rodekohr in ERDAS Software Forum
Edi,
Landsat data are stored in 16 bit format because the maximum data value exceeds 255. Sometimes the maximum values are anomalies, sometimes they represent definable items on the earth or in the atmosphere. If you look at the histogram of each band of the Landsat data you can see how skewed the data may be and if the max values are a couple of spurious points or a cluster of something real.
When using RESCALE you have two options to set up the rescale table: Min-Max is the default. If Min-max is used a linear stretch is used. If the histogram is extremely skewed then a bad stretch will result with clustering of data on one end of the data scale or the other.
If you use the Std. Dev. option the stretch will be based upon the mean of the values rather than the extremes, thus eliminating the problem with skew. You can set a wider range of SD (the default is 2.0). A value of 3.0 will minimize the clustering of values on the min max ends of the stretch.
Another alternative is to use the LUT stretch method. This stretch will use the Lookup Table that you can assign by using the Multispectral - General Contrast options. Make each band as sharp and clear as you can using these tools and then SAVE the table for the band. Now if you use the LUT Stretch program the output will be in 8-bit format and look exactly like it does on the screen.
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Re: Image data format conversion
By Donn Rodekohr in ERDAS Software Forum
Edi,
Another though on this subject -- realize that when you rescale or LUT stretch data from 16 bit to 8 bit that the data are no longer suitable for any type of classification or statistical analysis. It is now a picture that represents the image.
If you are planning on using the Landsat data for any analytical purpose, e.g., classification, NDVI, etc., you should keep the 16 bit format and get over appreciation of the original 8 bit format. The software doesn't care and any analysis will be statistically verifiable.
If you sole intent is for an image backdrop for viewing purposes then go ahead with the rescale or stretch.