You can create a mosaicked image that is displayed from a mosaic dataset, comprised of a number of input rasters that are often overlapping. The mosaic method defines how the mosaicked image is created from these input images.
For example, when using the By Attribute method, the values in an attribute field are used to sort the images, and when using Closest to Center, the image closest to the center of the display is positioned as the topmost image in the mosaic. Additionally, the mosaic operator allows you to define how to resolve the overlapping pixels, such as choosing a blending operation.
When working with a mosaic dataset, there are two places to modify the mosaic method. If you're editing the mosaic dataset and you need to set the default mosaic method (or limit the options), modify the mosaic dataset properties. This permanently alters the mosaic dataset. If you're viewing the mosaic dataset as a data source and not modifying it for other users, you can change the mosaic method from the image layer. This way, you only impact your display and do not permanently impact other users when they are viewing or accessing the mosaic dataset.
Mosaic methods
The default method (North-West) is used when a user accesses the mosaic dataset, but any number of mosaic methods can be used. You can modify the mosaic method and control the default and disable methods used with the mosaic dataset by modifying the default and allowed mosaic methods.
To see the effect of the various mosaic methods, consider the following example mosaic dataset composed of 12 individual rasters:
Refer to this diagram when reviewing the explanations for the individual methods below.
The mosaic methods include the following:
Closest to Center—Enables rasters to be sorted based the ZOrder, then PixelSize, and then by a default order in which rasters that have their centers closest to the view center are placed on top.
In this example, 12 rasters are displayed using the Closest to Center mosaic method. The center raster (pink) is completely displayed, and the remainder of the mosaicked image is composed of the remaining rasters. As you pan and zoom the mosaicked image, it changes as other rasters become closest to the center within the display extents.
Closest to Nadir—Enables rasters to be sorted by the ZOrder, then PixelSize, and then by the distance between the nadir position and view center. This is similar to the Closest to Center method but uses the nadir point to a raster, which may be different than the center, especially for oblique imagery.
Closest to Viewpoint—Orders rasters based on the ZOrder, then PixelSize, and then by a user-defined location and nadir location for the rasters using the Viewpoint tool.
In this example, the area indicated by the red arrow is zoomed in. This location comprises six overlapping rasters. Using the Viewpoint window tool, you can see those rasters displayed under the blue arrows, which represent the viewpoint selection.
By Attribute—Enables raster ordering based on ZOrder, then PixelSize, and then by a defined metadata attribute and its difference from a base value.
In this example, the chosen attribute field contains letters of the alphabet, and it's sorted in ascending order.
North-West—Enables raster ordering by ZOrder, then PixelSize, and then by the shortest distance between the center of a raster to the northwest position.
In this example, the raster in the northwest corner is used completely to create the mosaicked image.
Seamline—Cuts the raster using the predefined seamline shape for each raster, using optional feathering along the seams, and orders images based on the ZOrder and then the SOrder fields in the attribute table.
In this example, the seamline (in blue) is used to create the mosaic method, and the sort field is the same as the By Attribute method example above.
When setting the Seamline mosaic method, set the mosaic operator to Blend to allow some blending (feathering) to occur along the edge of the seamlines. The blending width is a value defined in the seamline attribute table.
Lock Raster—Enables a user to lock the display of single or multiple rasters based on the ObjectID.
In this example, four footprints are selected, so the mosaicked image is created from only four rasters.
None—Orders rasters based on the order (ObjectID) in the mosaic dataset attribute table.
In this example, the mosaicked image is generated based on the order of the rasters in the attribute table.
When using a date field with the By Attribute mosaic method, the base value must be entered using one of the following formats:
- yyyy/MM/dd HH:mm:ss.s
- yyyy/MM/dd HH:mm:ss
- yyyy/MM/dd HH:mm
- yyyy/MM/dd HH
- yyyy/MM/dd
- yyyy/MM
- yyyy
Mosaic operators
A mosaic operator is used to define how overlapping cells will be resolved in the mosaicked image. Mosaic operators include the following:
- First—The overlapping areas will contain the cells from the first raster dataset listed in the source.
- Last—The overlapping areas will contain the cells from the last raster dataset listed in the source.
- Min—The overlapping areas will contain the minimum cell values from all the overlapping cells.
- Max—The overlapping areas will contain the maximum cell values from all the overlapping cells.
- Mean—The overlapping areas will contain the mean cell values from all the overlapping cells.
- Blend—The overlapping areas will be a blend of the cell values that overlap; this blend value relies on an algorithm that is weight based and dependent on the distance from the cells to the edge within the overlapping area.
- Sum—The overlapping areas will contain the total sum of the cell values from all the overlapping cells.
It is recommended that you set Blend when using the Seamline mosaic method.
Sorting order rules
The sorting order of the images is impacted by the pixel sizes of each image, and it can be impacted by the ZOrder field in the attribute table or the SOrder field (when the Seamline mosaic method is used).
The ZOrder value is used in determining how the rasters will be mosaicked together when using the Closest to Center, North-West, By Attribute, Closest to Nadir, or Closest to Viewpoint mosaic methods. With these mosaic methods, the rasters are sorted by their ZOrder value first. The ZOrder value can only be modified by someone with the ability to edit the mosaic dataset's attribute table.
Generally, lower values push forward and higher values push backward. For example, if there are three values, 10, 0, and -10, the higher value (10) becomes heavier and has a lower priority when creating the mosaicked image, so it may be covered up by a lower ZOrder value (0 or -10). The lower the value, the more important and lighter it becomes, so it's displayed on top of a higher ZOrder value (-10 is on top of 0). A null ZOrder value is treated as 0. Another way to think about this order of display value is the higher number is displayed first and the remaining rasters are displayed over the preceding raster in descending order, so the lowest numbered raster is on top. You can also think of it as weight, in which the lightest value floats to the top.
SOrder works the same way as ZOrder except that it only applies to the Seamline mosaic method.
Rule orders
The lists below describe the rules regarding how each of the mosaic methods orders the data in a mosaic dataset. While the mosaic order descriptions listed in the section above were generalizations, these descriptions detail each possible scenario and how the images are ordered accordingly.
The following rules apply to the By Attribute, Closest to Center, Closest to Nadir, Closest to Viewpoint, and North-West methods:
- A spatial query defines the potential images.
- The minimum and maximum pixel size values are used to define the potential images.
- The ZOrder field value is examined and the images are prioritized.
- The cell size tolerance factor is used to refine the priority of overlapping imagery.
- The current scale is compared with the low pixel size values to refine the priority of overlapping imagery.
- The qualified images are selected and sorted according to the mosaic method (for the By Attribute mosaic method, the sorting order can be ascending or descending).
- The overlapping area is resolved by the mosaic operator.
For the Lock Raster method, all rules are overridden, except the mosaic method.
For the None option, no sorting rules are applied. The rasters are ordered as they are in the attribute table.
For the Seamline method, the following rules apply:
- A spatial query defines the potential images.
- The minimum and maximum pixel size values are used to define the potential images.
- The ZOrder field value is examined and the images are prioritized.
- The cell size tolerance factor is used to refine the priority of overlapping imagery.
- The qualified images are selected and sorted according to the mosaic method.
- The overlapping area is resolved by the mosaic operator.