Postmile Services

Welcome to the Postmile Services Postmile Query Tool

(This window can be dismissed by clicking the close button () at right, and retrieved with the button above the top-right corner of the map.)

The Postmile Query Tool enables you to explore California's Linear Reference System. You can validate postmiles, convert between postmiles and geo-coordinates, and determine the state odometer associated with a postmile or geo-coordinate. This window enables you to view a collection of help pages explaining what this tool is and how to use it.

Help topics

This page references data in the Postmile Web Service. The data is provided by the California Department of Transportation.

What is a Postmile?

A postmile is the way that a specific location on a state or federal route is specified within the Linear Reference System. In a linear reference system, locations are indicated by a distance along a route. Calibration postmiles represent locations that have been visually identified using aerial photography (National Agriculture Imagery Program—NAIP) and marked on the route. Other postmiles are interpolations between the calibration postmiles.

Postmile specification with County=SAC, Route=50, Route Suffix=S, Postmile Prefix=L, Postmile Value=1.234 and Alignment=R

A postmile has three principal components: a County, a Route and a Postmile Value. The county is specified by its 2- or 3-letter county code (e.g., SAC for Sacramento, SB for Santa Barbara).

The route is identified by a 1-3 digit number, and a Route Suffix Code for those routes that require one.

The Postmile Value measures the distance, in miles, from the start of the route, or from the point at which the route enters the county. Thus, Postmile Values reset to zero each time the route crosses a county border. (Exception: when the border between two counties meanders, so that the route crosses the same border several times, the Postmile Value continues to increase as though no boundary had been crossed, and is marked by a Postmile Prefix Code of "D" — duplicate postmile at meandering county line.)

The Postmile Value may be accompanied by a Postmile Prefix Code (immediately before the postmile value) and/or a Postmile Suffix Code (immediately after the postmile value).

The Route Suffix Code, Postmile Prefix Code and Postmile Suffix Code are referred to as "qualifiers." Not all postmile specifications require their presence. However, where they are needed, they must be present for the postmile specification to be valid. Sometimes, two postmile specifications, representing two distinct geographic locations, differ only in whether or not a single qualifier is present. For example, the two postmiles ALA 84 23.133 and ALA 84 R23.133 differ only by the presence of the "R" Postmile Prefix Code, but they represent two locations along Route 84 separated by about 880 feet. (In other cases, two postmiles differing in one qualifier can represent locations 10 miles apart.) For this reason careful attention to correct qualifiers is necessary. The next page lists the values that these qualifier codes can take, and what they mean.

Statewide Odometer

The Statewide Odometer is an alternate indicator of position within the linear reference system. The Statewide Odometer measures the distance of the location along a route from the beginning of the route, without regard for county boundaries.

The Postmile specification uses measures that are local to each county (and Postmile Prefix codes) so that realigning a route (modifying the path that a route takes) has minimal impact on the specifications of locations downstream of the segment that was changed. This is important because of the cost to change all the physical postmile paddles that line the routes. In the Statewide Odometer, on the other hand, any change in the path of the route affects the odometer specification for all points downstream of the change, across the entire state.

Qualifiers in a Postmile Specification

Route Suffix

If a route suffix is needed, it will be one of the two letters:

S — Supplemental
U — Unrelinquished

If a route suffix is included, there must be a space between it and the rest of the specification.

Postmile Prefix

If a postmile prefix letter is needed, it will be one of the following letters:

C — Commercial Lanes
D — Duplicate postmile at meandering county line
G — Reposting of duplicate postmile at route end
H — Realignment of D Mileage
L — Overlap postmile
M — Realignment of R mileage
N — Realignment of M mileage
R — First realignment
S — Spur
T — Temporary connection

Postmile Suffix

Sometimes the two directions of a major route separate and follow substantially different paths. When this happens, the postmile is determined independently for each direction. In that case a postmile suffix letter is required to indicate the side of the road to which the postmile pertains:

L — Left (South on South-to-North routes, West on West-to-East routes)
R — Right (North on Soutth-to-North routes, East on West-to-East routes)

Input Controls

Postmile Fieldset

This group of radio buttons controls whether the tool is configured to validate single postmiles ("Point" mode) or a line between two postmiles on the same route ("Line" mode).

This checkbox controls whether the components of the postmile(s) can be edited/viewed in separate drop-down menus, or whether each complete postmile specification is treated as a single text string.
If checked, there is a row of controls for each of the three components of a postmile: the County, the Route (and the sometimes required route suffix), and the postmile value (with options for the postmile prefix code and postmile suffix code).
If unchecked, there is a single text input field to accept or display the Postmile Specification.
In line mode, the controls for County, Postmile, and Postmile Specification (but not for the Route) are doubled so as to represent two postmiles.

The County drop-down menu lists California's 58 counties by county code and name. If a route is selected in the Route drop-down menu, then this menu will only show those counties that contain the route. In "Line" mode, there are two menus, labeled 1 & 2. A selection in either menu restricts the other menu to those counties that have a route in common with the selected county.

Image of Route and Route Suffix code drop-down menus

The route drop-down menu lists all the routes by number. When counties are selected in the county drop-down menus, this list is restricted to routes that pass through the selected counties. This grouping of controls also contains a drop-down menu for the Route Suffix qualifier, which some routes require.

Image of Postmile Prefix code drop-down menu, Postmile Value text input, and Alignment Code drop-down menu

This group of controls includes:

a drop-down menu for the Postmile Prefix qualifier,
a text input to accept the Postmile value, and
a drop-down menu for the Postmile Suffix qualifier.

This button initiates validation of the postmile specified in the other controls.
If the controls specify a valid postmile, then the location of the postmile is marked by a postmile paddle marker . An information window is also shown above the marker, describing the postmile and the corresponding Statewide Odometer. Paddle markers are only representative of location on the route and do not suggest or imply the location of an actual, physical paddle marker.

This button resets the values of all controls in the Postmile fieldset. It also removes all postmile paddle markers and route segments from the map, and clears the message in the validation message area above the map.

Input Controls (cont'd)

Geo-Coordinates Fieldset

Image of input text fields for Latitude and Longitude

These input controls enable the user to enter the latitude and longitude of a location for which a postmile needs to be identified.

The information for each coordinate can be entered in the degrees, minutes, seconds and cardinal compass point format, or as a signed decimal number of degrees. The format is determined by the Image of Deg/Min/Sec/Card checkbox checkbox that appears below the coordinate input controls.

Changing the coordinate values moves the pushpin marker to the indicated location.

In "Line" mode, there two sets of latitude and longitude fields, labeled "1" and "2", and two pushpin markers, and . Each set of input fields reflects and controls the position of its associated marker.

This button finds the postmile closest to the geographic location indicated by the coordinates. A postmile paddle icon marks the location of the postmile, and a line is drawn from the push-pin icon to the postmile paddle icon.

The specification of the postmile found this way is displayed in the controls of the Postmile fieldset.

In "Line" mode, the postmile specifications for the postmiles closest to the two geo-coordinate markers are inserted into the controls for the two postmiles. If the two postmiles lie on the same route, then the pair are validated as a line.

Input Controls (cont'd)

Other Controls

Image of checkbox with label: View County Boundaries, Routes and Calibration Postmiles

This check box appears above the top left corner of the map window. When checked, additional map layers are made visible, depending on zoom level.

At low zoom levels (when the map covers a large geographic region) county boundaries are drawn in black.

At higher zoom levels, the routes in the linear reference system are drawn in red.

At still higher zoom levels, the calibration postmiles are drawn as green discs, accompanied by the numerical portion of their postmile value (the qualifiers are not shown). Also, at these higher zoom levels the left and right sides of the routes are drawn independently (whether or not the linear reference system aligns the two sides independently). When this happens, the right side of the route is drawn in red, and the left side is drawn in blue.

At very high zoom levels these overlays again become invisible.

This text field and button appears above the top right corner of the map window, just to the left of the "Close Infowindows" button and the "Help" button. These controls enable the user to move a geo-coordinate marker to a location by specifying an address or the intersection of two routes. If the search string entered can be identified as a location in California, then the marker is moved there and the nearest postmile to that location is determined (as if the user had clicked the map in that location). In "Line" mode, the marker that is moved is determined by which input control in the Postmile fieldset or the Geo-Coordinates fieldset last had focus.

This button appears above the top right corner of the map window, between the "Search Address" control and the "Help" button. Clicking this button will close the information windows associated with all paddle markers visible on the map. This is useful when exploring the candidates returned by a failed postmile validation. Clicking on a candidate in the validation message will open the information window(s) of the corresponding postmile paddle marker(s). Clicking the Close Infowindows button enables you to clear the map of infowindows so you can clearly see which postmile paddle marker(s) participate(s) in the selected candidate.

This button appears above the top right corner of the map window. Clicking this button makes this help window visible (if it is not already) and sets the page to the table of contents.

Controls & Map Markers

Postmiles

Valid postmiles are represented on the map using markers with the postmile paddle icon .

Clicking on a postmile marker will show the information window for the postmile, and populate the postmile controls with the specification of the postmile at the marker's location.

In "Line" mode, if the marker represents a candidate for the first or second postmile of a line pair, the icon will show the number or , and clicking on it will populate the controls for the corresponding postmile.

Double-clicking a postmile marker will move the push-pin marker to the postmile's geographic location (and thus populate the geo-coordinate fields with the postmile's location).

Geo-Coordinates

The push-pin icon marks the location on the map represented by the coordinates in the geo-coordinates fieldset. This marker may be dragged with the mouse to move it to another place on the map. The geo-coordinate controls will change value to reflect the current position of the marker.

Clicking on the push-pin marker will determine the recognized postmile that is closest to the location of the marker, populate the postmile controls with it, and validate it (same as clicking the button in the geo-coordinates fieldset.) If the location of the postmile is different than that of the push-pin marker, a line will be drawn connecting the two markers.

In "Line" mode, the set of postmile controls that is populated is determined by which of the two markers is clicked (they are labeled and ).

Clicking on an open area of the map itself has the same effect as moving a push-pin marker to that location and clicking on it. In "Line" mode, the pushpin marker that is moved and the postmile fields that are populated is determined by the input control that last had focus: moving the focus into any input control for a postmile specification will cause the controls for that postmile to be populated with the postmile that is next found from geo-coordinates by clicking on the map.

When starting off and clicking anywhere on the map for the first time in "Line" mode, the controls for postmile 1 will be populated. Pushpin 2 will then appear on the map and will need to be double clicked in order to populate postmile 2's controls. Depending on the zoom level, pushpin2 may appear on top of pushpin1. To resolve this, use the + button at the bottom to zoom in closer.

How to: Point Mode

With the "Point" radio button selected, the query tool is in "Point Mode", which deals with an individual postmile representing a point geographic location on California's system of highways.

To Validate a Postmile

Enter the postmile specification in the "Postmile" fieldset, and click . If you entered a valid postmile, a marker with postmile paddle icon will show the location. An information window will also appear above the marker giving both the postmile specification and the state odometer for the postmile. The pushpin marker will also move to the location of the postmile, so that the geo-coordinates display will contain the latitude and longitude of that location.

If the postmile specification is not recognized by the Postmile Web Service, a message will appear to that effect above the map. In some cases, the web service can identify candidate postmiles. These are valid postmiles that differ from your specification only in the values of one or more qualifiers. When candidates are identified, they are listed in the message above the map, and they are located on the map with postmile paddle icons. You can populate the postmile controls with one of the candidates by clicking on its specification in the validation message (each is a hyperlink), or by clicking on the postmile marker for the candidate on the map. (Clicking either will also open the information window for the postmile paddle marker of the candidate. In conjunction with the Close Infowindows button you can explore the candidates.)

To Determine the Postmile for a set of Geo-Coordinates

Enter the geo-coordinates into the Latitude and Longitude input fields in the Geo-Coordinates fieldset (or drag the push-pin marker to the location on the map) and click

. This puts the pushpin marker

at the specified coordinates, then puts the postmile paddle marker at the nearest recognized postmile. A line will be drawn between the push-pin marker (representing your geo-coordinates) and the paddle marker (representing the nearest postmile).

Clicking on the map will move the pushpin marker to the click point, and display the nearest recognized postmile with the postmile paddle.

To determine the Geo-Coordinates of a Postmile

Validate the postmile by any of the methods above, then double-click on the postmile paddle. This will move the pushpin marker to the postmile location, and put the geo-coordinates in the latitude and longitude fields.

How to:Line Mode

With the "Line" radio button selected, the query tool is in "Line Mode", which deals with pairs of postmiles representing a segment of a route in California's system of highways.

To Validate a Postmile Pair

Enter the postmile specifications in the "Postmile" fieldset, and click .

The two postmiles must represent two locations on the same route (this is enforced in the "Components" controls by having only one set of controls for the route.

If you entered two valid postmiles, two markers showing the postmile paddle icons and will show the beginning and end of your segment, and the route between them will be marked in Sea Green. (Any parts of the route that are no longer under the aegis of the California Department of Transportation, or that overlap a dominant route in the system, will cause gaps in the marked route.)

If either or both of your postmile specifications are not recognized, then a message to that effect will appear above the map. This message may include candidate postmile pairs that differ from your specifications only in the values of one or more qualifiers. If so, postmile paddle icons will mark the positions of the candidates for each postmile. Clicking on a candidate pair in the message will populate the postmile controls, and pop up the information windows for the corresponding markers. Clicking on a postmile paddle icon labeled 1 or 2 will populate the controls for the corresponding first or second postmile.

When starting off and clicking anywhere on the map for the first time, the controls for postmile 1 will be populated. Pushpin 2 will then appear on the map and will need to be double clicked in order to populate postmile 2's controls. Depending on the zoom level, pushpin2 may appear on top of pushpin1. To resolve this, use the + button at the bottom right to zoom in closer.

To Validate a Segment by Geo-Coordinates

Enter the geo-coodinates for the ends of the segment in the Geo-Coordinates fieldset. Then click on the in the Geo-Coordinates fieldset. This will determine the postmiles for the ends of the segment. Of course, you can also determine the coordinates of the ends by dragging the corresponding pushpin markers.

To determine the Geo-Coordinates of the Segment Ends

After validating the segment, double-click on the postmile paddle icons shown at each end of the segment to move the push-pin markers there, then read the latitude and longitude displayed in the Geo-Coordinates fieldset. Alternatively, click the in the Postmile fieldset. This will revalidate the segment and move both geo-coordinate markers to their respective postmile locations.

For further information

The following is a document (Adobe Portable Document Format — PDF) where you can learn more about Postmiles and California's Linear Reference System:

Caltrans Postmile System (PDF)