Understanding the Image layer

The Image layer is a component of the mosaic dataset layer when it's added to ArcMap. It is used to control the rendering of the dynamically mosaicked image of the mosaic dataset. Modifications you make to the Image layer properties do not affect the mosaic dataset, only how it is rendered at the time you are viewing it. The Image layer is just like a raster layer—you can change the display properties, such as changing the renderer, band combination, or enhancement, as well as some specific properties, such as modifying the mosaic methods and viewing the properties of the dynamically mosaicked image.

The Image layer is not optimized for speed, but it is dynamic, and any changes made to the mosaic dataset are reflected in the Image layer. When you are using a mosaic dataset, any processing required to generate the mosaicked image is done on the local desktop, whereas if the mosaic dataset is served, and you are connecting to the image service, the processing to generate the mosaicked image is done on the server.

When there are problems with the mosaic dataset, such as the boundary not having been generated or the functions defined in the function chain not generating a valid output, the Image layer may not be checked, and it will appear with a small red .

Right-clicking the Image layer in ArcMap displays the shortcut menu with the following options:

• Zoom To Layer—Sets the display extent to the extent of the Image layer.
• Zoom To Source Resolution—Sets the display scale to view the raster data at its optimum resolution.
• Visible Scale Range—Allows you to set or clear a visible scale range. When the scale of the data frame is outside the layer's visible scale range, the layer will not draw.
  • Set Minimum Scale—Sets the minimum visible scale for the layer to display.
  • Set Maximum Scale—Sets the maximum visible scale for the layer to display.
  • Clear Scale Range—Clears the minimum and maximum scale ranges.
• Export Data—Allows you to export the mosaicked image and save it as a raster dataset.
• Properties—Opens the Layer Properties dialog box.

Image layer properties

To access the Layer Properties dialog box of the Image layer, right-click the Image layer and click Properties.

The Image layer properties of the mosaic dataset are somewhat unique and are very similar to the layer properties of the image service serving the mosaic dataset. Through the Display tab, you can change the transmission compression. A highly compressed image is transmitted faster than an uncompressed image; however, the image quality may not be as good.

• None—No compression is applied to the imagery, which provides the highest quality but results in the maximum volume of data transfer across the network.
• LZ77—An efficient lossless compression method recommended for imagery with similar pixel values (discrete data), such as scanned maps or results of classifications.
• JPEG—An efficient compression method that can often compress imagery by about three to eight times with little degradation of the image quality. When choosing the JPEG method, you can also edit the quality by typing a value from 0 to 100. A value of 80 tends to retain image quality while providing approximately 8x compression.
• LERC—An efficient lossy compression method recommended for data with a large pixel depth, such as float, 32-bit, 16-bit, or 12-bit data. When choosing this method, you need to specify the quality value, which represents the maximum error value that is applicable per pixel (not an average for the image). This value is specified in the units of the mosaic dataset. For example, if the error is 10 centimeters and the mosaic dataset is in meters, enter 0.1.

  LERC compresses better (5 to 10 times) and faster (5 to 10 times) than LZ77 for float data and is better with integer data. When using integer data, and the error limit specified is 0.99 or less, LERC behaves as a lossless compression.

The Mosaic tab is unique to the mosaic dataset (or an image service created from a mosaic dataset). Since the mosaicked image can be created from a number of input rasters, there are two settings to assist you in viewing the mosaicked image you need. The mosaic method defines how the mosaic is created from different rasters, such as By Attribute, where a field containing values is used to sort the images, or Closest to Center, where the image closest to the center of the display is on top.

The mosaic methods include

• Closest to Center—Enables rasters to be sorted based on the ZOrder, then PixelSize, and then by a default order where rasters that have their centers closest to the view center are placed on top.
• Closest to Nadir—Enables rasters to be sorted by the ZOrder, then PixelSize, and then by 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.
• By Attribute—Enables raster ordering based on the ZOrder, then PixelSize, and then by the defined metadata attribute and its difference from a base value.
• North-West—Enables raster ordering by the ZOrder, then PixelSize, and then by the shortest distance between the center of a raster and the northwest position.
• 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.
• Lock Raster—Enables a user to lock the display of single or multiple rasters based on the ObjectID.
• None—Orders rasters based on the order (ObjectID) in the mosaic dataset attribute table.

The mosaic operator allows you to define how to resolve the overlapping cells, such as choosing a blending operation.

The mosaic operators include

• 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 along the edge of each raster dataset in the mosaicked image. By default, the edge is defined by the footprint or the seamline for each raster.
• Sum—The overlapping areas will contain the total cell values from all the overlapping cells.

Both First and Last minimize the amount of data that is read; therefore, they will be the fastest methods. Min, Max, Mean, and Blend require the overlapping cells to be read, so these methods can be slower.

The Status tab presents the image properties of the mosaicked image, including the extent, number of rows and columns, number of bands, pixel depth, and compression information.