I have several hundred aerial imagery raster tiles (in ecw format) that I have loaded into a mosaic dataset. I would now like to 'replace' or 'overlay' an older aerial imagery layer with this dataset. Ideally I would like to merge them into one mosaic to save space and make it easier to manage.

However it is not a simple case of removing the old tiles by identifying which ones have updates (e.g SP21NE with new SP21NE) as the old aerial imagery layer has been stored as one large .ecw file and not as individual tiles in a mosaic dataset.

Could anyone suggest a work around please? Would it be best to use the updated imagery to 'clip' the old one and then merge them together in the mosaic dataset?


I would look into Synchronizing Mosaic Datasets, this should add the newer imagery on top of the older imagery.

You can then change the mosaic method you use, to sort by a date field when the image was captured.


Just to be clear, the 'old' raster(s) is/are NOT in a Mosaic dataset currently, and the old raster is in one large (seamless) raster. The 'new' rasters are tile-based and have already been loaded into a Mosaic dataset.

Here is my recommended solution:

  1. Create an 'old' Mosaic dataset from the 'old' seamless raster using the tile boundaries from the new Mosaic so that the tile boundaries match.
  2. Once you have two distinct Mosaic datasets (one 'old' and one 'new') then you can use the Synchronizing mechanism as described above.
  3. Lastly, you can replace the original seamless imagery layer with a layer pointed at the updated 'old' Mosaic after the tiles have been synchronized.

An alternative solution would be to use Raster Math to:

  1. Merge the tile-based updates into a single raster
  2. Create a 'mask' raster where new tiles are present in the merged result
  3. Null out the mask areas from the original single imagery layer
  4. Add the raster with the updates and the masked original imagery layer

The results will be a single imagery layer with the updates "knocked into" tile areas where they are present. Of course, then you could create a new Mosaic and load the imagery into it so that you don't have this problem in the future.

Lastly, keep in mind that by loading the imagery into a mosaic you are simply trading one set of difficulties for another -- is it best to manage a large single ECW or a mosaic where tile boundaries could shift over time?

  • Thanks, just to clarify how do I use the boundaries from the new mosaic dataset to create the old mosaic dataset? – Canningmister Aug 21 '14 at 8:45
  • Review the Build Footprints tool in toolbox > DataManagement > Mosaic dataset. This tool will create (internal) polygon footprints for each raster tile used in the "new" mosaic. Next, create that old mosaic dataset and use the "Import Mosaic Dataset Geometry tool" -- which conveniently copies the footprints from the tiled mosaic dataset into the one you just created. This way, both mosaic datasets will end up sharing footprints on a per-tile basis. – JasonInVegas Aug 21 '14 at 23:51

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