Profile developers guide

Technical reference for BRouter profile scripts

The tag-value lookup table
Context-Separation
Predefined variables in the profile scripts
Operators of the profile scripts
Syntactic Sugar
The initial cost classifier
The priority classifier
The elevation buffer ( From Poutnik’s glossary )
Technical constraints
Developing and debugging scripts
Lookup-Table evolution and the the major and minor versions
Other resources

The tag-value lookup table

Within the routing data files (rd5), tag information is encoded in a binary bitstream for the way tags and the node tags each.

To encode and decode to/from this bitstream, a lookup table is used that contains all the tags and values that are considered for encoding.

For each tag there are 2 special values:

<empty> if the tag is not set or the value is empty
unknown if the value is not contained in the table

Each value can have optional aliases, these alias values are encoded into the same binary value as the associated primary value.

A profile must use the primary value in expressions, as aliases trigger a parse error. E.g. if there is a line in lookups.dat file:

bicycle;0001245560 yes allowed

then a profile must use bicycle=yes, as bicycle=allowed gives an error.

The numbers in the lookup table are statistical information on the frequency of the values in the map of Germany - these are just informational and are not processed by BRouter.

Context-Separation

Way-tags and Node-Tags are treated independently, so there are different sections in the lookup table as well as in the profile scripts for each context. The special tags: ---context:way and ---context:node mark the beginning of each section.

An exception from context separation is the node-context, where variables from the way-context of the originating way can be accessed using the way: prefix. For the variable nodeaccessgranted there’s an additional legacy-hack to access it as a lookup value without prefix:

if nodeaccessgranted=yes then ...

while in the general case the prefixed expressions are variables:

if greater way:costfactor 5 then ...

In the profile scripts there is a third context global which contains global configuration which is shared for all contexts and is accessible by the routing engine.

The variables from the global section in the profile scripts are read-only visible in the way and node sections of the scripts.

Predefined variables in the profile scripts

Some variable names are pre-defined and accessed by the routing engine:

for the global section these are:
- 11 elevation configuration parameters:
  - downhillcost
  - downhillcutoff
  - downhillmaxslope
  - downhillmaxslopecost
  - uphillcost
  - uphillcutoff
  - uphillmaxslope
  - uphillmaxslopecost
  - elevationpenaltybuffer
  - elevationmaxbuffer
  - elevationbufferreduce
- 3 boolean mode-hint flags
  - validForBikes
  - validForFoot
  - validForCars
- 2 variables to change the heuristic coefficients for the 2 routing passes ( <0 disables a routing pass )
  - pass1coefficient
  - pass2coefficient
- 3 variables to influence the generation of turn-instructions
  - turnInstructionMode 0=none, 1=auto-choose, 2=locus-style, 3=osmand-style, 4=comment-style, 5=gpsies-style, 6=orux-style, 7=locus-old-style
  - turnInstructionCatchingRange default=40m
  - turnInstructionRoundabouts default=true generate explicit roundabout hints
- variables to modify BRouter behaviour
  - processUnusedTags default=false
    
    If an OSM tag is unused within the profile, BRouter totally ignores the tag existence. Skipping unused tags improves BRouter speed. As a side effect, the tag is not even listed in the route segment table nor the table exported as CSV. Setting it to true/1, Brouter-web Data page will list all tags present in the RD5 file.
for the way section these are
- turncost
- initialcost
- costfactor
- uphillcostfactor
- downhillcostfactor
- nodeaccessgranted
- initialclassifier
- priorityclassifier
for the node section this is just
- initialcost

Operators of the profile scripts

The profile scripts use polish notation (operator first).

The assign operator is special: it can be used only on the top level of the expression hierarchy and has 2 operands:

assign <variable-name> <expression>

It just assigns the expression value to this variable (which can be a predefined variable or any other variable, which in this case is defined implicitly). The expression can be a complex expression using other operators.

All other operators can be used recursively to an unlimited complexity, which means that each operand can be a composed expression starting with an operator and so on.

All expressions have one of the following basic forms:

<numeric value>
<numeric variable>
<lookup-match>
<1-op-operator> <operand>
<2-op-operator> <operand> <operand>
<3-op-operator> <operand> <operand> <operand>
A numeric value is just a number, floating point, with . as decimal separator. Boolean values are treated as numbers as well, with 0 = false and every nonzero value = true.
A lookup match has the form <tag-name>=<value>, e.g. highway=primary Only the primary values can be used in lookup-matches, not aliases. The <empty> value is referred to as an empty string, e.g. access=
1 Operand operators are:
- not <boolean expression>
2 Operand operators are:
- or <boolean expression 1> <boolean expression 2>
- and <boolean expression 1> <boolean expression 2>
- xor <boolean expression 1> <boolean expression 2>
- multiply <numeric expression 1> <numeric expression 2>
- divide <numeric expression 1> <numeric expression 2>
- add <numeric expression 1> <numeric expression 2>
- sub <numeric expression 1> <numeric expression 2>
- max <numeric expression 1> <numeric expression 2>
- min <numeric expression 1> <numeric expression 2>
- equal <numeric expression 1> <numeric expression 2>
- greater <numeric expression 1> <numeric expression 2>
- lesser <numeric expression 1> <numeric expression 2>
3 Operand operators are:
- switch <boolean-expression> <true-expression> <false-expression>
  
  So the switch expression has a numeric value which is the true-expression if the boolean expression is true, the false-expression otherwise.

Syntactic Sugar

To improve the readablity of the profile scripts, some syntactic variations are possible:

if then else: if can be used instead of the switch operator, if the additional keywords then and else are placed between the operators:

if <boolean-expression> then <true-expression> else <false-expression>
Parentheses: each expression can be surrounded by parentheses: (<expression>)

Please note that the profile syntax, due to the polish notation, does not need parentheses, they are always optional. However, if there are parentheses, the parser checks if they really match the expression boundaries.
or-ing lookup-matches: the pipe-symbol can be used as a short syntax for lookup matches where more than one value is accepted for a key: highway=primary|secondary|tertiary
additional = symbol for assign operations:

assign <variable-name> = <expression>
boolean constants: true and false can be used instead of 1 and 0

Please note that the tokenizer always expects blank space to separate symbols and expressions so it is not allowed to place parentheses or the = symbol without separating blank space!

The initial cost classifier

To trigger the addition of the initialcost, another variable is used: initialclassifier - any change in the value of that variable leads to adding the value of initialcost.

Initial cost is used typically for a ferry, where you want to apply a penalty independent of the length of the ferry line.

Another useful case may be an initial cost for bicycle mounting/dismounting, having set an initialclassifier for ways without bicycle access, with high initialcost. For backward compatibility, if initialclassifier = 0, it is replaced by the costfactor.

The priority classifier

priorityclassifier is a BRouter numerical parameter calculated for ways and used for generation of pictogram/voice navigation instructions.

Higher values means the more significant (noticeable) way, as far as it can be predicted from OSM data.

To avoid a navigation instruction flood, it was decided that the instructions are provided only if:

You are supposed to turn at a crossroad/junction and some other ways having the same or higher priorityclassifier value.
You are supposed to go straight ahead and some other ways having the higher priorityclassifier value.

The elevation buffer ( From Poutnik’s glossary )

With related 3 internal BRouter variables:

elevationpenaltybuffer
elevationmaxbuffer
elevationbufferreduce

the Elevation Buffer is BRouter feature to filter elevation noise along the route. It may be real, or caused by the artefacts of used SRTM elevation data.

From every elevation change is at the first place cut out amount 10*up/downhillcutoff per every km of the way length. What remains, starts to accumulate in the buffer. IF cutoff demand of elevation per length is not saturated from incoming elevation, it is applied on elevation remaining in the buffer as well.

E.g. if the way climbs 20 m along 500 m, and uphillcutoff=3.0, then 10*3.0*0.5 km = 15 m is taken away and only remaining 5 m accumulates. But if it climbed only 10 m on those 500 m, all 10 m would be swallowed by cutoff, together with up to 5 m from the buffer, if there were any.

When elevation does not fit the buffer of size elevationmaxbuffer, it is converted by up/downhill[maxslope]cost ratio to Elevationcost portion of Equivalentlength. up/downhillcostfactors are used, if defined, otherwise costfactor is used.

elevationpenaltybuffer - default 5(m).

The variable value is used for 2 purposes
- with buffer content > elevationpenaltybuffer, it starts partially convert the buffered elevation to ElevationCost by up/downhillcost
- with elevation taken = MIN (buffer content - elevationpenaltybuffer, WayLength[km] * elevationbufferreduce*10 The up/downhillcostfactor takes place instead of costfactor at the percentage of how much is WayLength[km] * elevationbufferreduce*10 is saturated by the buffer content above elevationpenaltybuffer.
elevationmaxbuffer - default 10(m)

is the size of the buffer, above which all elevation is converted to Elevationcost by up/downhill[maxslope]cost ratio, and - if defined - up/downhillcostfactor fully replaces costfactor in way cost calculation.
elevationbufferreduce - default 0(slope%)

is rate of conversion of the buffer content above elevationpenaltybuffer to ElevationCost. For a way of length L, the amount of converted elevation is L[km] * elevationbufferreduce[%] * 10. The elevation to Elevationcost conversion ratio is given by up/downhill[maxslope]cost.

Whether up/downhillmaxslope or up/downhillmaxslopecost is used as conversion ratio depends on whether the elevation was accumulated below or above the slope threshold values defined in up/downhillmaxslope.

Example: Let’s examine steady slopes with elevationmaxbuffer=10, elevationpenaltybuffer=5, elevationbufferreduce=0.5, cutoffs=1.5, up/downhillcosts=60.

All slopes within 0 .. 1.5% are swallowed by the cutoff.

For slope 1.75%, there will remain 0.25%.

That saturates the elevationbufferreduce 0.5% by 50%. That gives Way cost to be calculated 50% from costfactor and 50% from up/downhillcostfactor. Additionally, 0.25% gives 2.5m per 1km, converted to 2.5*60 = 150m of Elevationcost.
For slope 2.0%, there will remain 0.5%.

That saturates the elevationbufferreduce 0.5% by 100%. That gives Way cost to be calculated fully from up/downhillcostfactor. Additionally, 0.5% gives 5m per 1km, converted to 5*60 = 300m of Elevationcost. Up to slope 2.0% the buffer value stays at 5m = elevationpenaltybuffer.
For slope 2.5%, there will remain 1.0% after cutoff subtract, and 0.5% after the buffer reduce subtract.

The remaining 0.5% accumulates in the buffer by rate 5 m/km. When the buffer is full (elevationmaxbuffer), the elevation transforms to elevationcost by full rate of 1.0%, i.e. 10 m/km, giving elevationcost 10*60=600 m/km.

Technical constraints

The costfactor is required to be >= 1, otherwise the cost-cutoff logic of the routing algorithm does not work and you get wrong results.
The profile should be able to find a route with an average costfactor not very much larger than one, because otherwise the routing algorithm will not find a reasonable cost-cutoff, leading to a very large search area and thus to long processing times.
Forbidden ways or nodes must be treated as very high cost, because there is no forbidden value. Technically, values >= 10000 for a (way-)costfactor, and >= 1000000 for a nodes initalcost are treated as infinity, so please use these as the forbidden values.
Ways with costfactor >= 10000 are considered as if they did not exist at all.
Ways with costfactor = 9999 are considered as if they did not exist during route calculation, but the navigation hint generator takes them into account.

Developing and debugging scripts

For developing scripts, the brouter-web web-application is your friend. You can use that either online at https://brouter.de/brouter-web or set up a local installation.

brouter-web has a window at the lower left corner with a Profile and a Data tab. Here, you can upload profile scripts and see the individual cost calculations per way-section in the Data-tab.

For profile debugging activate assign processUnusedTags = true to see all present OSM tags on the Data tab, not just those used in the tested profile.

Lookup-Table evolution and the the major and minor versions

The lookup-table is allowed to grow over time, to include more tags and values as needed. To support that evolution, it carries a major and a minor version number. These numbers are also encoded into the routing data files, taken from the lookups.dat that is used to pre-process the routing data files.

A major version change is considered to always break compatibility between the routing datafiles and the lookup table.

A minor version change keeps the routing data files and the lookup-table compatible in both directions, using the following rules:

if the data contains a key that is not contained in the lookup tables, it is ignored
if the data contains a value that is not contained in the lookup tables (but its key is known) that value is treated as unknown
if a profile uses a key that is not present in the data, it sees empty (=unset) values for that key
if a profile uses a value that is not present in the data, lookup matches for that value are always false.

For a minor version change it is required that tags are only appended at the end of the table (or replace one of the dummy tags located between the way-tags and the relation pseudo-tags), and that values are only appended at the end of the value lists. This is because the routing data files address tags and values by their sequence numbers, so changing sequences would produce garbage data.

Other resources

See Poutnik’s glossary as a complementary source about various profile internals.