Overview of MAP messagesDavid Kelley, SubCarrier Systems Corp. (SCSC)davidkelley@iTSware.net

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Presentation Outline• MAP & SPaT• The Basics• The Generic Lane Explained• Multiple Lane Uses• A Typical ConnectsTo Example and SPaT• Pedestrian Crossing Examples• Putting it all Together• Creating MAP/SPaT, Key Steps• A “good” MAP has...• Further Resources

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MAP and SPaT... Are tied at the hip, you must have one to use the other.

• When used together, they operate within the PSID called “Intersection safety and awareness” (0x84)

• MAP is also used for all general purpose road geometry needs in all of DSRC in the US (and in EU).

• There are 15 messages in the J2735 / J2945/x effort, and SRM SSM are also used with SPaT for preemption-priority.

• The SAE is at this time creating a recommended practices document to address deployment questions (J2945/10).

The goal of this briefing is to provide a basic understanding of MAP Content and Structures.

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The Basics• MAPs represent sets of related lanes;

Each lane is a closed polygon with attributes• MAPs precisely translate between WGS-84 LLH to a

localized XYZ orthogonal system• MAPs support a 1 cm resolution• MAPs are compressed messages

(using a delta encoding system)• MAPS require (“flattening”)

before any application use• BSMs can easily be placed on a

flattened MAP for application use

One MAP message

One Intersection

sMany Lanes

1..32

1..255

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The Basics, 2• SPATs represent one or more Intersection States

Typically just one for now (Key concept: 1 intersection ≠ 1 signal controller when needed)

• Each Intersection State holds The active Lanes (Key concept: More than just signal times) The active and future movements and times, Time is in UTC

One SPAT message

One Intersection

Inter. States

1..4

1..32

One Intersection State

A Movement State

Signal Group IDState

Times

1..255

Timing Data

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A typical MAP, with some BSMs• If a MAP msg is rendered

into a simple graphical format and BSMs are placed on it; you get this.

• This is a 2009 edition fragment.

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The Generic Lane Explained, 1

Based on early deployment experience, a common format was developed. Many separate needs were combined to make the “generic lane” used now.• Represents 8 different “lanes types” with a

common structure.• Includes: Motor Vehicles, Ped Lanes,

Medians, Bicycle Lanes, Trains, etc. • Each Lane Type has the attributes it

needs to describe its use case.• And within each lane, further attributes are

allowed, at a node and spanning nodes.• The system is expandable to provide for

attribute growth.

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The Generic Lane Explained, 2

A bit more detail on contents...• Key concept:

Each Generic Lane has a context of default values for many items • A local anchor point is always used, and very precise

LLH -> XYZ conversion factors determined

• A sequence of offset nodes describes the polygon (next slide)• A set of Lane-level Attributes and Node-level Attributes • A list of ConnectsTo entries to link the stop line to the next lane and

to the signal groupID

• And a set of allowed maneuvers that can made at the stop line.

Left Side Not shown

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Generic Lane, 3 Nodes in the Polygon

• Nodes describe the centerlines.• All Nodes are offsets from the

prior node (or anchor point).• The Lane is described from the

stop line backwards.• Widths can be changed as

required (a linear taper along the segment), or use the default.

• The angle of each new point is projected from the last.

• End skew is added when needed.

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The Generic Lane Explained, 4

• Attributes, two examples: Do Not Block, (a segment use) Curb at Step Off (a node use)

• The ConnectsTo Concept (example in a moment)Key Concept: Link between Lane & SPAT

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Multiple Lane UsesRecall:

A MAP Is Static, And contains all lane details deemed to be relevant

The SPAT Is Dynamic, All time of day details (hence which lane description is to be used at a given time of day)

This is used to have the SPaT enable: Reversible Lanes Time of Day Parking Lanes Right on Red and other

Crossing lanes that vary with timeKey takeaway: SPaT contains more than simply the signal controller state

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A typical ConnectsTo ExampleA common left turn with an admissive ahead lane and a protected left turn lane.

• Note: a connectsTo can connect to lanes in another intersection

Example Left Turn LaneWith two ConnectsTo entries

Group A &Group B

Group B

Egress LaneEg

ress

Lan

e

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Pedestrian Crossing Examples• Crosswalk Lanes are typically just 2-node lanes.• Features like a safety island are added

using the same basic constructs as lanes (uses same attribute system).

• Considerable style variations exists at ends.• Scrambles (Barnes Dance) are

also easily supported.• Crosswalk Lanes also have attributes

to support ADA accessibility issues.

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Aside: Several subtle issueswith angles are seen here

Tapers for complex lane shapes• Widths and Tapers are Often needed for Crosswalks• Here is a two-point lane in NY

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Putting It all together• A typical MAP message has

One Intersection withMany Lanes (each with a LaneID, and GroupIDs, in its ConnectsTo)

• A typical SPaT message has One Intersection (may not be one signal controller)

Active Movements (each with TimeMarks for GroupIDs

• MAP and SPaT are part of the Intersection safety PSID / App • Signal Request and Signal Status (not covered) provide priority and preemption

support. The Intersection Collision msg provides safety alerts. • To place a BSM on a map; Subtract anchor point from BSM data, scale by local rate. • On-line graphics from XML spy: http://dsrc-tools.com/xml/dsrc_70/dsrc/

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Creating MAPs/SPaTs, the key steps

1. Gather map source materials, signal timing plan, and local signage.

2. Capture Centerlines / Widths / Stop Lines (with suitable GIS tools, KML, JSON, etc, and not ASN)

3. Add all Needed Attributes (weak tool support at this point, often added by hand)

4. Extract XML/JSON in full floating point precision (and do datum transformations, capturing the process details)

5. Reduce to ASN(and Merge Attributes if not done above)(filter unused lane content as needed here)

6. QC the work !!

• Perhaps do one/two in work shots of such tools as side slides.

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A “Good” map has these testable features

• First and foremost: Only the content you need for your applications,Not every lane with every possible detail.

• Well formed ASNusing the correct compression choices (well encoded)

• Correctly placed stop lines and angles• Small Inter-lane to lane gaps under ~50cm• Spatial accuracy in the range of 20~40 cm absolute position

• Traceability to the source maps, the datum used, and coordinate transformation used, and the author

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Pragmatic Deployment Advice for the Challenge

• Use the official PSID: 0x84 (or one of the test ones).• Use the experimental Intersection ID value ranges.• Use the numbering practices being developed by SAE.• MAP & SPaT & BSM will need differential corrections

(RTCM) to work for some applications. • Decide who is signing each message (local or central).• Add local content when you feel you need it,

we are all still learning.

• When in doubt, just ask.

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Resources, 1

The SAE J2735 DSRC Message Set Standard Available at: https://www.sae.org/standardsdev/dsrc/ In XML image form: http://dsrc-tools.com/xml/dsrc_70/dsrc/

Good GIS Background Materials Understanding Coordinate and Datum issues:

https://www.ngs.noaa.gov/datums/newdatums/FAQNewDatums.shtml Understanding Precise Distances with Vincenty Methods:

https://www.movable-type.co.uk/scripts/latlong-vincenty.html A useful Google Datum conformance paper:

http://www.hydrometronics.com/downloads/Web%20Mercator%20-%20Non-Conformal,%20Non-Mercator%20(notes).pdf

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Resources, 2

3rd Party Tools Used In this Presentation (all but one are free) ANS.1 viewer: https://www.obj-sys.com/products/asn1ve/ (free limited feature edition)

Global Mapper: www.globalmapper.com/ (commercial tool)

QGIS: https://qgis.org/en/site/ (open source)

Leidos “ISD Message Creator” tool:https://webapp2.connectedvcs.com/ (open source)

Additional Commentary on Proper MAP/SPaT usage http://dsrc-tools.com/map-spat/ (contents draft)

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Thank you

David KelleyITS Programs Manager, SubCarrier Systems Corp. (SCSC)626-513-7715 (Office)davidkelley@iTSware.net