I would like to ask for clarification about how ele=* should be used for a helipad located on top of a building, because we have different interpretations in our local discussion at Spanish group.
ele=* for the landing surface elevation above sea level?ele=* be avoided in that case, using only building-related height/level tags?Only this one.
Why not add ālevelā?
Just in case both values are known.
Not ready to dig into previous discussions ā Iād call ele an immediate attribute of the feature that is mapped. Regardless of precision, the top of a tower must not have the same ele as its base. By no means, consumers should be concerned with calculating ele by adding xĆtimes level (how much is that?) to the mapped ele to get at the actual ele, that is the locally accepted altitude above sea level.
I would expect ele= to be of helipad not of something else, as it would be added to helipad
So actual elevation of helipad (top of building)
ele= can be also added to building, I guess
Yes.
You could also use location=roof but I wouldnāt bother, if the helipad is within a building area, then itās always going to be rooftop.
Yes and you would therefore expect the building=*'s ele= + height= to equal to the aeroway=helipadās ele=. In fact this is how we are able to tag the rooftop elevation, even without a helipad.
I think youāre looking for the term altitude.
As far as I understand, the key āeleā is used for terrain elevation, not for artificial objects at a certain altitude. In Germany, some helipads on buildings are mapped using the alt and height keys.
The wiki recommends using the key ele for airports and heliports, but for some reason ele is not mentioned for helipads.
I want to point out the danger of using the key āeleā for artificial elements with altitudes different from the surrounding elevation. The most obvious danger is the replacement of the depth key with ele to map objects below sea level or below the level of a river. And although itās a strange example, I imagine someone with strange logic mapping=
historic=wreck + ele=-45 for a shipwreck at sea or historic=wreck + ele=355 for a shipwreck in a river or a lake that is located at a depth of 20 meters from the surrounding elevation at 375 meters above sea level.
Itās important to remember that a helipad is part of a building=
In case of buildings and other man_made=* structures that stand on the ground, tag the elevation of the surrounding ground, not of the structure itself (i.e. do not tag the elevation of the tip of a tower; instead, set ele=* to the elevation of the ground level, and add the height with the tag height=*
The key ele It has some useful suffix such as= ele:min=* ele:max=* ele:natural=* If you ultimately wish to use ele, a suffix should be added to indicate that the value is not indicating the elevation of the terrain or ground, but the value corresponds to the altitude of an element above ground level and I would propose ele:agl=*or use the same key that has been used for elevated helipads in other regions: alt.
why you think so?
ele=*specifies elevation (height above sea level) of a point, in metres.
yes, also looking at somehow similar situations, the documentation for man_made=cross mentions explicitly to add ele with the ground elevation as value: Tag:man_made=cross - OpenStreetMap Wiki
your quote says it: elevation.
The ground is describing the root height the cross is based on. These =helipad are based on the roofs.
In aviation, elevations are reported for takeoff/landing, thus not necessarily the ground. I checked the AIS for an elevated =heliport , and the AIP records it at the top.
This can be seen from Wiki, an FAA source, and its links LAPD Hooper Heliport - Wikipedia
You didnāt explain anything by repeating āelevationā. ā(height above sea level) of a pointā reads to me as for the point itself, not the ground below.
It should be highlighted ātag the elevation of the surrounding groundā in the last paragraph of the lede refers to āstructures that stand on the groundā. Therefore if itās not based on the ground, this doesnāt apply.
why you think that āelevationā applies always to base ground level?
probably because these crosses are ground-standing and someone wanted to express that you do not map elevation of top of a cross
I asked people more familiar with topic, and so far response is unanimous āobviously, elevation of heliport would be elevation of landing area - not base terrain levelā
They linked for example https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_150_5390_2D_Heliports.pdf doing it in the more formal way
1.8.46
Touchdown and Liftoff Area (TLOF).
A load-bearing (generally paved) area normally centered in the FATO, on which the
helicopter performs a touchdown or liftoff.(ā¦)
1.8.16
Heliport Elevation.
The highest elevation of all helicopter landing areas (TLOFs) within the heliport,
expressed as the distance above mean sea level (MSL).
(EDIT: obviously, we are not obligated to follow FAA definitions - but it demonstrates that it is also used in this way)
I believe that osm is a digital representation of the real world, from which technical terms and their definitions are drawn to represent it as faithfully as possible; osm has relied on the wiki as a technical guide to standardize mapping, and although there are still gray areas that can lead to misinterpretation of tag usage, itās necessary to say that the wiki remains the ultimate guide. Thereās a constant evolution where technical rigor and terminology have been displacing and replacing colloquial usage, so the wiki aims to be more than just a dictionary. However, weāre still stuck on terms like āelevation,ā where itās equated with anything thatās high. In a geographic database, elevation is a terrain attribute, not an arbitrary measure of the vertical position of an artificial object.
The wiki mentions (I can also be selective, which reinforces my point of view 
):
āThis is mainly intended for mountain peaks but could also be used for elevation of airport runways and many other objects.ā AND:
āIn case of buildings and other man_made=* structures that stand on the ground, tag the elevation of the surrounding ground, not of the structure itself (i.e. do not tag the elevation of the tip of a tower; instead, set ele=* to the elevation of the ground level, and add the height with the tag height=*).ā In this case, the elevated helipad is located on the roof of a building and is part of it.
In aeronautics, ICAO defines elevation as the vertical distance from the highest point of the landing area to mean sea level. However, this is an operational definition for aviation, and I wonder if there might be confusion due to the extrapolation of definitions, with attempts to apply the definition of a term from one discipline to another, such as cartography, where data must be structured differently.
It has been suggested that the formula elevation + building height = helipad elevation is valid, but this is a fundamental conceptual error. In cartography and geodesy, the correct relationship is: Elevation (ground) + Height (structure) = Altitude. In this context, the only valid elevation for āeleā is that of the ground. Forcing the sum of the height of an artificial object within an elevation variable to obtain an elevation value is technically incorrect.
The wiki mentions where to primarily use āeleā and also mentions where not to use it. If, ultimately, āeleā is not equivalent to the term āelevationā used in cartography to refer to points on the Earthās surface, and instead the definition of āeleā is left open to the mapperās subjective interpretation, we could save space on the wiki by removing everything related to EGM96 and other technical standards that wonāt be considered, and simply stating: use āeleā as you wish. However, if technical rigor is desired, it must be respected that the elevation ends where the construction begins.
P.S. Please do not use the excuse that Wikipedia is not the best source of information; the OSM wiki has thousands of links to it.
I would say that in this case:
ele on building, if tagged (preferably not tagged) would be for ground surfaceele on helipad which stands on rooftop, not ground would be for its actual surfaceĀÆ\_(ć)_/ĀÆ
maybe it is, tell that to say FAA for start
It seems to me that it is far from being as clear-cut as you claim (EDIT: obviously, we are not obligated to follow FAA definitions - but it demonstrates that it is also used in this way)
definitely not in all of cartography, for start
I understand your point of view and the regulatory support you give to the manuals and circulars of the aviation authorities. However, I find it problematic that you are selective with this documentation and only point to what reinforces your opinion. I sincerely hope you are not suggesting taking the FAA/ICAO as a reference or authority to determine what constitutes elevation, since although aeronautics and cartography are disciplines that share technical terms, their definitions and objectives are often opposed to each other.
I donāt know if you are aware that the FAA and ICAO also have manuals and circulars for Obstacles and Obstructions where practically everything that protrudes even slightly from the ground has an elevation value (according to their operational standards). This includes: poles, towers, antennas, elevated water tanks, buildings, power lines, etc. I donāt think we want to turn OSM into an inventory of obstacles for aviation, displacing cartographic rigor with that of air navigation. I imagine an absurd scenario where all OSM elements that protrude from the ground and are near airports and heliports carry the key ele:* simply because there is an FAA/ICAO manual or circular that says it must be so.
oh definitely I am not claiming THAT, in the same as we are not obligated to follow Wikipediaās definitions.
but I consider it as a counter to claim that āelevationā always and solely refers to altitude of the ground and cannot refer and must not refer to altitude of the say helipad on rooftop
What does āAlthough the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.ā mean then? Below, āIn geometry and geographical surveys, altitude helps create accurate topographic maps and understand the terrainās elevationā is stated as āterrainās elevationā, not simply āelevationā.
The examples given are āaircraft in flight or a spacecraft in orbitā. In contrast, āelevation is mainly used when referring to points on the Earthās surfaceā can be interpreted as objects based on the surface, thus including raised ones.
A paywalled paper discussed this, which you can circumvent in well-know websites On the use of elevation, altitude, and height in the ecological and climatological literature | Oecologia | Springer Nature Link
Anecdotally, for common language https://english.stackexchange.com/questions/16610/which-to-use-altitude-or-elevation-in-regards-to-height-above-sea-level
As two of us disagree with altitude= , and you disagree with ele= , how about height:absolute= from GIS? But according to the above, ele= is really allowed softly, while altitude= is opposed strongly.
āFor features that occur at a constant height above a datum, regardless of ground elevation, such as airplanes. In addition, you can add a vertical exaggeration to the absolute height. This way, you can make differences in the elevation more prominent.ā Define height characteristics for layersāArcGIS Pro | DocumentationAgain, āelevationā is used for both. āGround elevationā / āterrain elevationā is clarified. When ele= already exists, why not use it?
According to Key:ele - OpenStreetMap Wiki it specifies height above sea level of the feature tagged.
OSM is not usually technically correct, technical terms that mean specific things in specific fields of study get abused into different meanings when applied as OSM tags, moreso when applied across different global regions.
If ele= (not elevation, just the ele tag) on a building= is the building base above sea level, and height= is how far it extends upwards, then the building rooftop measurement above sea level must be ele + height. Iām not talking about āelevationā or āaltitudeā here, just the raw OSM tags.
I think having more technical rigour where it can improve our work is a good thing, so if anything it sounds like more tagging development and a new tagging proposal about how to record elevations, altitudes, heights and depths of features might help.
Iād love a better way to specify the altitude/elevation of the rooftop, but we have no direct tag for this.