Introduction
In AIXM 5.1.x vertical dimensions of obstacles can be coded as the vertical extent of each obstacle part and the elevation on top of each part:
- the vertical extent is the vertical distance from the bottom to the top of the part itself; on a single part obstacle the vertical extant is the height from the ground to the top of the obstacle. It is coded in the VerticalStructurePart.verticalExtent attribute.
- the elevation is the MSL elevation of the topmost point of each part. Depending on the part's geometry, the elevation is coded in ElevatedSurface.elevation attribute for polygons, in the ElevatedCurve.elevation attribute for line obstacles or in the ElevatedPoint.elevation attribute for line obstacles.
AIXM 5.2 Change
This specification explains how obstacle elevations and heights shall be coded in AIXM.
Height versus Elevation
With regard to vertical dimensions of obstacles, the current AIXM 5.1(.1) model allows coding for each VerticalStructurePart:
- the elevation (MSL) at the top of the obstacle part, using the VerticalStructurePart.horizontalProjection choices (ElevatedPoint, ElevatedCurve, ElevatedSurface)
- the vertical dimension of each obstacle part, using the VerticalStructurePart.verticalExtent property
Workaround to determine obstacle height
As stated above the current model does not have an attribute that could be used directly for coding the height of the obstacle (AGL). However:
- for obstacles that have a single VerticalStructurePart, the height (AGL) is equal to the VerticalStructurePart.verticalExtent (assuming that the obstacle sits on the ground and it is not floating in the air)
- for obstacles that have multiple parts, the height (AGL) of the obstacle as a whole may be deduced in most cases using the following algorithm:
- identify the VerticalStructurePart that has the maximum top elevation (MSL, at the top) - let's call it "max elevation". This is also the overall obstacle maximum elevation at the top.
- identify the VerticalStructurePart that has the minimum base elevation, by subtracting its verticalExtent from its elevation (MSL) at the top - let's call it 'min base elevation'
- the difference between 1 (max elevation) and 2 (min base elevation) will give the overall vertical extent of the obstacle, which is also equal to its overall height (AGL)
Once the model provides an attribute for obstacle height it is recommended to code the height directly rather than to apply the above workaround.
As shown above the vertical extent is just the vertical dimension of a part regardless of its position relative to the ground. This behaviour can be used to define multi-part obstacles with 'floating' elements, where an aircraft (e.g. an helicopter) can operate beneath.
Single Part Obstacles
A single part obstacle is a vertical structure that is coded as only one part,
- A single part obstacle can have the shape of a point (e.g. an antenna), a line (e.g. a fence) or a polygon (e.g. a forest). However, being a single part obstacle, it can have only one vertical extend/elevation. It is the task of the data originator to decide if this simplified coding is appropriate or if a more refined multi-part obstacle coding should be used.
- The top elevation of the obstacle is encoded with the corresponding horizontalProjection of its part element.
- The height of the obstacle is encoded as verticalExtent of the verticalStructurePart. Since the vertical structure can have only one vertical extent, the maximum vertical extent is used, as if it was the same for all points even in line or polygon obstacles.
- The unit of measurement is required and shall be coded as an attribute of the elevation element.
- In relation to the elevation a verticalDatum and a verticalAccuracy may be coded in addition
Point Obstacle (e.g. antenna)
Line Obstacle (e.g. fence)
Note
Polygon Obstacle (e.g. forest)
Multi-Part Obstacles
A multi-part obstacle is a vertical structure that is composed from multiple parts.
- Each part of the obstacle can have the shape of a point (e.g. an antenna), a line (e.g. a wall) or a polygon (e.g. a building)
- For each part the top elevation shall be encoded with the corresponding horizontalProjection of its part element
- For each part the verticalExtent may be encoded. Please note that for multi-part obstacles the verticalExtension
- The unit of measurement is required and shall be coded as an attribute of its element (elevation repectively verticalExtent)
- In relation to the elevation a verticalDatum and a verticalAccuracy may be coded in addition.
The example blow demonstrates the coding of
- the elevations P1 and P2 of the building and the elevation of the antenna on top
- the vertical extents P1 and P2 of the building and the antenna on top. In this case the verticalExtent is the height of the obstacle parts.
- the verticalExtent for part P1 reflects the height above ground level.
- the verticalExtend for part P2 does not reflect the height above ground level. (Note: P2 may be also not on top of the roof but on the side).
Please note that currently the heights P1 and P2 above ground cannot be coded. Coding of heights will be enabled with the introduction of AIXM 5.2
Beside construction of 'simple' buildings composed of several cubes, multipart obstacles allow the definition of more complex structures like bridges, power lines and cable cars, even with the possibility to fly under parts of them.
Powerline with same elevation of all parts
The figure is similar to a single part obstacle with line geometry, but this time encoded as three parts instead of one. All 3 parts have the same elevation.
Cable car with different elevation of its parts (maximum height AGL)
The figure shows a case where the height of P2 is considered to be the maximum height above ground level. The elevation of the parts is different.
Powerline across a valley
In this case it was intended to allow flights below the obstacle (e.g. a cable crossing a fjord).
- The verticalExtent of the pylons (P1 & P3) can be considered the height above ground level as they have point shape.
- The verticalExtent of P2 is considered to be the difference between the elevation of the highest and lowest point of P2.