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For marking a way's incline (or steepness/slope). In the case of roads, there is often a warning sign along the road.
The incline is most commonly given as percentage values (postfixed with a percent sign, %), e.g. incline=15%. Degree-values should only be used where they are common (postfixed with a degree sign, °), e.g. incline=10°.
Split the way at the ends of the steep section and add a tag incline=value% on the way with values as given above. The value should be given for the practical maximum incline on the steep section (i.e., the maximum incline that a vehicle/primary user could achieve), and not for the average incline between the nodes. See Talk:Key:incline.
The use of this tag has limited (if any) usage, as nodes do not have any direction. It may be used only on nodes that are used by a single way, but it does not mean that this is the inclination of the whole way.
To avoid problems that could occur when the node is shared by an intersection of ways, you should tag instead a segment-way between two nodes where the incline angle is effectively measurable.
Extremely short inclined segments (under about 1 meter on ways for vehicles, or below 10 centimeters for footways usable by wheelchairs) generally don't need to be tagged as they are not really significant barriers if the incline rate remains below about 50% (otherwise they are effectively "steps").
In case you don't know the exact value you can:
- use incline=up for ways going up the hill in the direction of the way, and incline=down for ways going down the hill in the direction of the way
- roughly estimate and do not mind if it is somewhat wrong. It would be helpful to add another tag, like FIXME=check incline.
Relationship between percent and degree values
A (very steep) rise of 1 meter vertically for every 1 meter horizontally can be represented as 100% or as 45°. In the UK this may be signed as '1 in 1'.
Important note: the % format is an exponential value compared to the ° format which is a constant value, 10°=17.63% incline and 30°=57.73% incline (not 52.89%).
Values can be converted as follows:
- ‹incline in %› = tan(‹incline in °› × pi/180) × 100
- ‹incline in °› = arctan(‹incline in %› / 100) × 180/pi
- The multiplication or division by the constant pi/180 is only required if the trigonometric functions work with radians units instead of degrees.
Common & extreme inclines
Common values for this tag are in the range −25% to 25% on roads. Ways with a steepness of under 10% are seldom signposted, except on motorways in mountainous regions (for sections longer than about 500 meters where an acceleration not controlled by braking may expose to danger, notably in curves or with other slower vehicles or with people walking along the road), but inclines of over 20% are rare and pure visual estimation of steepness often results in too big values.
Maximum known inclines on public streets is 37% at Canton Avenue - the steepest officially recorded public street in the United States. Standing upright in an incline of 52% (found in downhill pistes) can already be tricky, and not only because the shoes start to slip. The steepest groomed skipistes are around 100% (i.e., Piste de l'Aigle in Zinal, Switzerland, around 100%, or Harakiri in Mayrhofen, Austria, at 85%). Offpiste skiing takes place on even steeper terrain for experts only.
Long inclines in mountains are considered steep for bicycles when they reach 15% for a significant length. In sports competitions, they are challenging and frequently assigned a "category" according to the total length of steep incline. There's no international standard for this category, each competition decides the category and assigns specific points for the sportive challenge, and this categorization may change over time at each competition event; however the categorization is for a long route that include sections with variable inclines requiring more than a few minutes of efforts. So there's no permanent signs for these categories except during these competition events.
On the opposite, the categorisation of long steep inclines on pistes open for skiing is signed permanently with conventional colours marking their difficulty. These colours (typically from green to blue, red, and black in Europe) are tagged separately. See piste:difficulty=*.
One stated maximum incline for a footway usable in a wheelchair has been 6%.