Source mosaic datasets
Creating a new mosaic dataset
It is recommended to set the coordinate system for each source mosaic dataset to be the same as the source data, to facilitate quality assurance/quality control testing.
Note that some single collections may include multiple projections (for example, in states that cross multiple UTM or state plane zones). It is recommended to either select the zone that covers the most datasets within the collection or use another projection, such as Web Mercator, for the mosaic dataset. The mosaic dataset performs projection on the fly, so the projection in which the mosaic dataset is managed does not need to be the same as that of the original data. Also note that any reprojection is always performed as a single step, from original source data to desired output projection, minimizing any resampling.
Adding rasters
Preprocessed orthophotos should be ingested using the Raster Dataset raster type. Before adding rasters, set the Cell Size Tolerance Factor (in mosaic dataset properties) to 999.
In the Add Rasters step, the Update Cell Size Ranges flag should be turned on when adding rasters into the source mosaic dataset.
It is recommended to not include the Update Overviews flag at this step. Overviews are typically recommended, but can be added later, after key metadata is entered into the attribute table.
See considerations on creating overviews below.
As an option, at this stage, records in the source mosaic can be selected and then merged. This will combine the selected records in the attribute table into a single record, which you may find desirable for the sake of simplification. This would typically be most applicable to use case C, when users have access to many different image collections.
Metadata
Metadata should be populated by adding appropriate fields to the attribute table. Follow the recommendations listed above at the time each source mosaic dataset is populated with data. Typically all data in one collection has the same metadata, so the simplest method for entering this metadata into the attribute table is to use the Calculate_Field tool to fill values for all fields.
Calculating cell size
It is not necessary to calculate cell sizes, since this was done in the Add Rasters step.
Refining geometry
This step from the Standard Workflow refers to x,y adjustments to improve the horizontal accuracy, but it is not typically required for preprocessed orthophotos.
Footprints and NoData
In the mosaic dataset properties, do the following.
- Set Clip to footprint? to YES, so any NoData areas are removed.
- Set Define NoData value? to NO, since the footprint is being used to manage NoData.
After these settings, different workflows are required for building the footprints depending on the data case.
Data case 1: Tiled orthophotos
Run the Build Footprints tool using the Radiometry option, with the number of vertices set to 8 (the default).
Data case 2: Multi-image mosaics in large files
If accurate polygons exist to define the valid extents of imagery within each file (for example, a polygon representing the county boundary), import those polygons into the footprint of the mosaic dataset using the Import Footprint or Boundary tool.
Otherwise, run the Build Footprints tool using the Radiometry option, and estimate a reasonable number of vertices to define the footprint, based on the footprint complexity; an example value would be ~100. These mosaic images often have a large amount of overlap, so using a large shrink distance is advised. Several items to note are as follows:
- The units for this setting are based on the coordinate system of the mosaic dataset, not pixels—so consider the resolution of your imagery (meters/pixel) when setting this shrink distance. (for example, with 0.5-meter data, a value of 50 meters = 100 pixels).
- If the data is compressed (for example, MrSID, ECW, JP2, or JPEG format), the original NoData values will have been altered by the compression process, so the Build Footprints process must be configured to ignore a range of NoData values:
- If the NoData is black (pixel = 0), adjust the minimum value to a few digits greater than zero (for example, try 4).
- If the NoData is white (pixel = 255), adjust the maximum value to a few digits less than 255 (for example, try 251).
Some experimentation will likely be required to determine the optimum settings. If you have a large collection of imagery, test with a small selection of images before running your full mosaic dataset.
Data case 3: Individual orthorectified image frames
If accurate polygons exist to define the valid extents of each image, import those polygons into the footprint of the mosaic dataset using the Import Footprint or Boundary tool.
Otherwise, run the Build Footprints tool using the Radiometry option with the number of vertices set to 4, and set the shrink distance so the footprint is reduced to be well inside the valid extents of each image. Several items to note are as follows:
- The units for this setting are based on the coordinate system of the mosaic dataset, not pixels—so consider the resolution of your imagery (meters/pixel) when setting this shrink distance (for example, with 0.5-meter data, a value of 10 meters = 20 pixels.
- The shrink distance must not be too large to lose overlap between images.
- If your source data files have a collar of nonimage data (such as fiducials and film annotation, if the source was scanned film), you will likely need to use a much larger shrink distance, for example, 200 pixels.
Some experimentation will likely be required to determine the optimum settings. If you have a large collection of imagery, test with a small selection of images before running your full mosaic dataset.
Refining radiometry
If color correction has already been applied to the data, changes to image radiometry should not be required.
Data case 1: Tiled orthophotos
If color correction has not already been applied, examine the imagery visually, and decide if color correction is important. If yes, color correction should be completed for each source mosaic dataset (for image-to-image matching). Then, it may also be desired to adjust the source mosaic datasets to achieve year-to-year matching. If you decide to apply color correction, refer to Color correcting a mosaic dataset in the ArcGIS help, and be sure to exclude areas of water, snow, or clouds.
Data case 2: Multi-image mosaics in large files
Color correction cannot be applied to correct problems within multi-image mosaics. If your data exhibits color or contrast problems, the individual images must be adjusted (by your data provider) prior to building the multi-image mosaics.
If you have multiple-year collections and would like to match tones and contrast from year to year, some limited color correction may be feasible, as mentioned in data case 1.
Data case 3: Individual orthorectified image frames
For this case, image-to-image color correction has typically not been applied. Recommendations are the same as those for data case 1.
Seamlines
Seamlines are not typically required except for data case 3: Individual orthorectified image frames. Examine your source mosaic visually, and if you find distracting mismatches between images (typically from tall buildings that exhibit apparent layover), the Mosaic dataset seamlines ArcGIS help topic should be helpful with regard to building seamlines using the Radiometry option.
Mosaic dataset properties
Recommended settings for the properties of each source mosaic dataset are described below. Any properties not mentioned may use default settings. Run the Set Mosaic Dataset Properties tool to set properties on the mosaic dataset.
- Transmission compression settings:
- Default Method: JPEG
- JPEG with recommended quality: 80
- Default resampling method:
- Bilinear Interpolation
- Download:
- If you want to allow users to download source raster data, the value for Maximum Number of Downloadable Rasters will limit the number of images. To disable download, set this value to 0.
- Set Blend Width to 0, since no blending is desired.
- Set Always Clip the Raster to its Footprint to Yes.
- Set Footprints May Contain NoData to No.
- Set Always Clip the mosaic dataset to its Boundary to No.
- Statistics are not required at the mosaic dataset level.
- Under Raster Information, set Source Type to Processed. This informs ArcGIS that the color correction and contrast enhancements have been applied, so an automatic stretch is not applied by the software.
After running Calculate Values to set properties, review the overview records in the attribute table to ensure they include the same key metadata as the rest of the image collection.
Overviews
Overviews are reduced-resolution views that are prebuilt to improve display speed at small scales.
If your imagery will be processed to create a raster tile cache, it is not necessary to create overviews.
Otherwise, if you have multiple source mosaic datasets, it is recommended to create overviews for each. Run the Define Overviews tool, and set the base pixel size to be approximately 1/500 of the width of a typical image file. For example, if each image covers 2 km, set overview base pixel size to ~4 meters. Set compression to JPEG, quality to 80, and resampling to Bilinear.
After overviews are defined, run Build Overviews.