Postmiles is used to display postmile locations on a map display, or to query for the purposes of finding a specific location reference by county, route, and postmile. Postmiles are coded with the district, county, route, postmile prefix (if any), postmile, and postmile suffix (if any). The source of data is a combination of the Caltrans TSN (Transportation System Network) database and the Caltrans Office of Data Services and Technology's Linear Referencing System (LRS), with updates performed by Caltrans District 5 GIS.

Two additional fields - PMInterval and AlignCode - should be understood by the user for effective use of this data set (refer to the Entity and Attribute Information section of this metadata).

PMInterval indicates the interval on which the postmile falls: 0.001, 0.01, 0.1, 1, 5, or 10. This field is handy for drawing a subset of postmiles at desired intervals, for example this Definition Query will tell ArcMap to draw postmiles only at 1 mile intervals:

"PMInterval" >= 1

Though the vast majority of postmiles fall on a 0.1 (tenth mile) or higher intervals, postmiles are included that fall on a lesser interval. Examples are the beginning or end of a route or county, or at an equation point. These postmiles can be identified through use of the "HwySegment" field that contains codes of "Begin County", "Begin Segment", "Mid Segment", "End Segment", and "End County". Begins and ends of routes are coded "Begin County" and "End County". A Definition Query that would tell ArcMap to draw postmiles only at 1 mile intervals or the begin of county is:

"PMInterval" >= 1 OR "HwySegment" = 'Begin County'

AlignCode indicates if the postmile is on a Right alignment, Left alignment, Right Side of an independent alignment, or Left Side of an independent alignment. Where TSN considers most state highways to have just a centerline, the underlying linework (LRS) that the postmiles are based upon has a line for each direction. In some cases the right and left lines are identical (where there is no median). In other cases the right and left are separated (where there is a median, or dual carriageways), but TSN treats the segment as a single centerline. In the last case the underlying linework has dual carriageways, and TSN treats the segment as an independent alignment (where there are independent right and left alignments). In some cases (especially for small-scale mapping), it may be appropriate to display postmiles for just one alignment (e.g. Right alignment), and only for the other side where TSN indicates independent alignments. In other cases (e.g. large-scale mapping), it may be appropriate to display postmiles for both alignments, regardless of how TSN treats the segment. This is an example of a Definition Query that would tell ArcMap to display postmiles at one mile intervals only on the right alignment where TSN indicates just a centerline, and on both right and left alignments where TSN indicates independent alignments:

"PMInterval" >= 1 AND "AlignCode" <> 'Left'

Similarly, only at 1 mile intervals or beginning of county, and not Left:

("PMInterval" >= 1 OR "HwySegment" = 'Begin County') AND "AlignCode" <> 'Left'

The Odometer field represent the actual distance in miles from the start of the highway to the postmile. This is in contrast to the postmile value, which no longer represents this value as each highway is realigned (and made longer or shorter) over time.