Questions about statistics when building a DEM mosaic.

[DavidMedeiros]

I'm trying to build a mosaic dataset for a collection of ASTERv2 DEMs and am running into an issue with the appearance of the mosaic.

Following the instructions in the link below I calculated statistics and built pyramids for my ASTERv2 tiles before adding them t the mosaic dataset. I then created overviews so I could preview the mosaic at full scale. But when I finished the resulting mosaic looks mostly gray, as though no stats had been calculated.

I've then tried to clac stats for the mosaic itself but the process never finishes. I've tried several times up to 91 hours for the longest attempt with no results. When I compare my ASTERv2 mosiac to a older ASTER mosiac we have which does look ok as a mosaic the only difference I see is that the input tiles to the older mosaic did not have statistics built first. You cans ee the difference in the attached PDF.

Any idea what I need to do to get this newer version to look like  the older version? Do I need to go back and remove statistics from the tiles before adding to the mosiac?

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009t00000039000000

I appreciate the help,
David

[JakeSkinner]

Hi David,

Depending on the amount and size of imagery that makes up your mosaic dataset, building statistics can take a long time.  To see features in some images or raster datasets, or to distinguish  them more easily from the surroundings, you may want to alter the  stretch applied to the histogram. In many cases, you can get the  enhancement you need by stretching based only on the pixel values within  the display and not using all the pixels in the raster dataset. This is  a dynamic range adjustment (DRA).  Try the following:

1.  In ArcMap, go to Windows > Image Analysis
2.  In the Image Analysis window, select the mosaic dataset, check DRA, and set the stretch to 'Std-dev' and resampling to 'Nearest Neighbor'

Does this improve the display of your mosaic dataset?

[DavidMedeiros]

Thanks Jake,

I'll give that a try for ArcMAP.  My problem is that what I really want is for students who are going to be evaluating the data in ArcCatalog to see the higher contrast images in the ArcCatalog Preview tab as well. That's why I was using the calc stats.

I think it took overnight, maybe 6-7 hours, to calc stats on the individual tiles in the mosaic, so I can't image it would take longer than 91 hours for the mosaic itself but maybe it's trying to do it to all of the overviews as well. I'm guessing it has something to do with the order I'm applying the overviews, stats, pyramids etc. And from looking at the original ASTER mosaic I have I can see that while stats were successfully built for the mosaic, they were not done for the tiles. So maybe I need to remove them from tiles first.

Hi David,

Depending on the amount and size of imagery that makes up your mosaic dataset, building statistics can take a long time.  To see features in some images or raster datasets, or to distinguish  them more easily from the surroundings, you may want to alter the  stretch applied to the histogram. In many cases, you can get the  enhancement you need by stretching based only on the pixel values within  the display and not using all the pixels in the raster dataset. This is  a dynamic range adjustment (DRA).  Try the following:

1.  In ArcMap, go to Windows > Image Analysis
2.  In the Image Analysis window, select the mosaic dataset, check DRA, and set the stretch to 'Std-dev' and resampling to 'Nearest Neighbor'

Does this improve the display of your mosaic dataset?

[JakeSkinner]

In ArcCatalog, go to Customize > ArcCatalog Options > Raster tab > Raster Layer tab.  What is your stretch and resampling set to there?

Also, you can speed up the calculation of statistics by specifying a skip factor in the Calculate Statistics tool.  This will give you a generalization of the statistics.

[DavidMedeiros]

In ArcCatalog, go to Customize > ArcCatalog Options > Raster tab > Raster Layer tab.  What is your stretch and resampling set to there?

Also, you can speed up the calculation of statistics by specifying a skip factor in the Calculate Statistics tool.  This will give you a generalization of the statistics.

I ran the clac stats tool with a skip factor (100 and then 50). The tool ran much faster (5 - 15 minutes), and although it seemed to finish successfully, there was still no change in how the mosaic previews and the statistics fields under Properties is still blank.

[JeffreySwain]

Did you restart ArcCatalog or just refresh the workspace?  Since you said that the statistics did not appear it seems that they are still not appearing.

[JustinShepard]

I'm having a similar issue. When I build the statistics in 10.1, for a mosaic dataset in a 10.1 file geodatabase, they don't appear to be stored by ArcMap. However, if I use a copy of the same rasters in 10.0 (10.0 fgdb and mosaic dataset) and calculate statistics for a mosaic dataset then the statistics are generated. I have tried refreshing the workspace and exiting ArcGIS completely. Here's the kicker, if I run this for a very small selection of the rasters (just 6 instead of thousands of files) then the stats work! If it didn't complete in 10.0 I would think that there was a data corruption issue in one of the rasters.

For the 10.1 mosaic dataset, in ArcCatalog the MinPS and MaxPS are blank even though the calculate statistics process completed without any errors.

Result message from calculating stats
***
Executing: CalculateStatistics J:<myfilepath>1 1 # OVERWRITE "<name>"
Start Time: Fri Oct 19 11:22:46 2012
Succeeded at Sat Oct 20 08:25:36 2012 (Elapsed Time: 21 hours 2 minutes 50 seconds)
***

When I tried an operation that would require stats.
***
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Stretch Renderer
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Statistics are not available. You can edit High/Low values or use custom statistics for minimum-maximum and standard deviation stretch.
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OK  
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***