Mosaic dataset

Question

Dear all,
           While creating a mosaic datset using 16bit data, I have found that I looks darker than the original image. When I applied a stretch and change the data type to 8bit I looks like original but to get it enable I have to remove that function.I relally cont understand what is happening with this tools are they really working or just behaving like working!!!!! Still if I keeps adding images I found huge difference in tone among images in mosaic dataset compared to original. These are pan images for information.
Will be very useful If some one help me through this. All what I want is my mosaic data should atleast look like original image, and I should be able to work with only selected images in Image server 10.

LuizMoura · Answer

After doing step #1 the raster strech only could be applied using Calculate Statistics (Data Management/Raster/Raster Properties/Calculate Statistics). Right-click in the Mosaic Dataset, then Modify and Build Item Pyramids and Statistics not seemed work.

PeterBecker · Answer

It is most likely what you are see is the effect of statistics having being computed on the datasets and then stretch being applied.

When you add a raster dataset to ArcMap, if statistics have been previously computed and included with the dataset, then ArcGIS by default applies a 2 standard deviation stretch renderer to the imagery. This has the effect of increasing the contrast of the imagery and for many 16bit datasets results in the data being visible vs seeing near black. This stretch can be turned off for a specific layer by going to the layer properties and changing the stretch to None. Alternatively if by default you do not want a stretch to be applied when adding imagery,then go to Customize, ArcMap Options, /Raster/, Stretch type and set this to None.

When you add a raster to a mosaic dataset a stretch by default is not applied to each individual raster. As a result it may appear to be darker. Multiple images may be added to a mosaic dataset and in many cases you do not want each image to have a separate stretch applied. When overviews for a service are built the system computes statistics for the complete mosaic dataset. When this mosaic dataset is then added to ArcMap either directly or via an Image service, ArcMap will again apply a default stretch. A typical example of where this of most use is when say adding multiple elevation datasets. IE the data is NonProcessed and by default you want ArcGIS to apply a stretch renderer to the complete mosaic dataset when is is added for display, so that the data can be seen. If you do not ArcMap to add a stretch based on the statistics of a Mosaic Dataset then you can set MosaicDataset Properties, /Defaults/, Is Preprocessed Data to Yes. IE the mosaic dataset includes images that have already been processed and so no additional stretch should be applied. Typically this would be used when creating a mosaic dataset of pre-processed imagery or when the mosaic dataset includes processes to color correct imagery.

If you have a case where you have a set of images and you want each to be stretched individually in the Mosaic Dataset there are two options:
1 - When Adding the Raster Datasets to the mosaic dataset, click on the Raster Type properties button and change the Processing Template to 'Stretch'. The type of stretch can be changed by going to the Functions tab and opening the Stretch\StretchFunction item that details the stretch to be applied to each image when added.
2 - If the data is already added, then use the 'Batch Edit Raster Functions' (Accessible by making a selection set of the rasters and then right click on the Footprints, Selection, Batch Edit Raster Functions). This will enable you to 'Insert' a function into the process chain of each raster. In this case you would probably want to add the 'Stretch' function with Standard Deviation stretch.
Note that when adding a stretch you can also define the bit depth of the output. Keep in consideration what the bit depth of the mosaic dataset is. You may want to specifically define the mosaic dataset as 8bit when you create it.

Doing either 1 or 2 above and then setting Is Preprocessed to Yes would result in you seeing the same result as if you had added each raster individually to ArcMap with the default stretch being applied.

JeffreySwain · Answer

Are you making the mosaic dataset 16 bit when you create it?  Also what is the output bit depth specified in the mosaic dataset?  Are you choosing to display them as 16 or as 8 bit unsigned?  I would recommend specifying the desired bit depth to start with and then consider what the output of the mosaic dataset is specified.  There may be issues encountered if the mosaic dataset if the bit depths are mismatched.