Time in a mosaic dataset
This topic applies to ArcEditor and ArcInfo only.
When you create a mosaic dataset, it is recommended that you add a time field to the attribute table. Time can be a year, such as 2010, or a date, such as 2010/01/02 or 2455198.50000 (Julian date), or it can contain a date and time such as 2010/01/02 15:30 or 2455199.14583.
Time can be stored using date, string, or numeric field types; however, there are limitations to the formatting depending on how you intend to use the field, which is discussed below.
Management of the mosaic dataset
There are two main ways to organize your mosaic dataset when using time: all your imagery in one mosaic dataset or one for each year with another to contain all years.
Single mosaic dataset configuration
Putting all the images into one mosaic dataset works when they all have similar extents and scales that don't require overview images or you don't plan to view the images based on time unless you are zoomed in to an appropriate scale. For example, if you have temperature images over the world, they tend to be at low resolution and cover the entire earth in one or just a few images. Therefore, if you zoom out to the full extent, you will still see an image.
You may need to check the minimum and maximum pixel sizes in the attribute table to be sure they all represent the correct range—for example, all starting at 0 (MinPS) and all ending at the same value (MaxPS).
The other example doesn't require overviews. For example, if you have many aerial images from different years, you might only want to allow a query when a user has zoomed in to an appropriate scale (allowing features to be seen clearly and not hitting the maximum number of rasters per mosaic limit). Here you could create the overviews from only the most recent images if you require overviews or use another lower-resolution dataset or service in place of the overviews.
In each of these cases, you will create a time field in the attribute table. If you don't have an image for each time interval, such as one for each year, you need to create start time and end time fields.
Multiple mosaic dataset configuration
If you are mixing collections of imagery into one mosaic dataset, you may want to manage each collection in a separate mosaic dataset, then create another mosaic dataset to contain the collections. This way, not only will the images be managed separately, but you can also create overviews for each collection and add them to the compiled mosaic dataset; therefore, the user will be able to see images for the requested time value at any scale.
In this example, you will create a mosaic dataset for each collection, such as one for each year. You will add a time field and populate it for each collection. You will create another mosaic dataset, then add the individual collections using the Table raster type. This will add each individual record from each input mosaic dataset and any of the additional fields you added. Make sure the time fields or other fields are created consistently; otherwise they won't be joined together properly.
If time is being used as part of a query, there are no restrictions on the formatting of time; however, the user will need to use a consistent format in the query as in the attribute table. For example, a query using "01-02-2010" will not return a value stored in a numeric field as 2010/01/02.
Using the By Attribute mosaic method
If a date field is being used with the By Attribute mosaic method, a user must enter an order base value in the following format:
- yyyy/MM/dd HH:mm:ss.s
- yyyy/MM/dd HH:mm:ss
- yyyy/MM/dd HH:mm
- yyyy/MM/dd HH
This should also be the same format used within the time field. Also, the By Attribute method only supports a field stored as a date or numeric field type, not a string.
When you choose By Attribute, you are defining a mosaic method, not a query. Therefore, the image that is displayed on top is either the closest in time to the order base value (if sorting ascending) or it will be the farthest away (if sorting descending). For example, suppose you have five images from 1957, 1973, 1991, 2002, and 2008; they all cover the same extent with the same cell size; and the order base value is 1980. If the order is ascending, the image on top will be 1973 because the difference between 1980 and 1973 is 7, which is the smallest difference. If the order is descending, the image on top will be 2008 because the difference between 2008 and 1980 is 28, which is the largest difference.
Using the Time Slider window
If time is being used within the Time Slider window, the date field is recommended; however, if you're not using a date field, the format of the field must adhere to the specifications in Supported time field formats. If the time information is stored in a date field, then, by default, the format for storing these values is inherited from the operating system's settings as described in Fundamentals of date fields.
The time slider is designed to display the information (or imagery) at an increment in time. If your mosaic dataset does not have an image for each interval, such as each year, no image will appear. Therefore, it is recommended you create and use start and end year fields; this way, no interval in time will be displayed without an image.