LandXML Feature Representation

LandXML is an XML-based format, so all features have both XML specific format attributes, as well as LandXML-specific format attributes.

This table lists the LandXML-specific format attributes.

Attribute Name	Contents
landxml_landxml_type	The LandXML element type that this feature belongs to. On a feature being written to LandXML, this will be the first place looked for determining what LandXML element type this feature should be written as. Possible values are those element types supported (as mentioned in the Format Overview section).
landxml_retwall_offset_ above	On a Surface Feature, determines how to interpret the vertical offset on the geometry.
landxml_retwall_offset_ right	On a Surface Feature, determines how to interpret the horizontal offset on the geometry.
landxml_element_id	An attribute which identifies this feature. It is used to track the hierarchy of elements within a LandXML document.
landxml_parent_id	An attribute which identifies the parent of this feature. It is used to track the hierarchy of elements within a LandXML document.
landxml_children_id	A comma-separated value attribute which identifies the children of this feature (by landxml_element_id value). It is used to track the hierarchy of elements within a LandXML document.
landxml_equipment_type	This attribute identifies the type of the equipment used for a survey. Only found on Survey feature types. Current possible values are "equipment_laser_details”,"equipment_instrument_details","equipment_gps_reciever_details","equipment_gps_antenna_details”.

In particular, landxml_element_id, landxml_parent_id, and landxml_children_id all work together to provide information on the hierarchy of elements within the original LandXML document. These attributes are bookkeeping attributes, in that they store information on the placement of elements within the original LandXML document, but are not properties of the elements themselves. That is, there is no element or attribute that corresponds to the landxml_parent_id attribute, but rather the placement of the element itself is recorded in these format attributes.

As an example of these attributes in action, consider the following LandXML fragment

<Parcels name="AirPorts">
   <Parcel name="AirPort_01">
      <Parcels name="Runways_01"/>
   </Parcel>
   <Parcel name="AirPort_02">
      <Parcels name="Runways_02">
         <Parcel name="Runway_02_01"/>
      </Parcels>
   </Parcel> 
</Parcels>

Here we see a Parcels feature, which contains two Parcel features, each of which in turn contains a single Parcels feature, and one of those embedded Parcels contains a single Parcel feature. The following features would be produced (with geometry and irrelevant attributes omitted for clarity).

Parcels
   name : Airports
   landxml_element_id : 1
   landxml_children : 2,4

Parcel
   name : AirPort_01
   landxml_element_id : 2
   landxml_parent_id : 1
   landxml_children : 3

Parcels
   name : Runways_01
   landxml_element_id : 3
   landxml_parent_id : 2

Parcel
   name : AirPort_02
   landxml_element_id : 4
   landxml_parent_id : 1
   landxml_children : 5

Parcels
   name : Runways_02
   landxml_element_id : 5
   landxml_parent_id : 3
   landxml_children : 6

Parcel
   name : Runway_02_01
   landxml_element_id : 6
   landxml_parent_id : 5

So within FME, these IDs can be used to reconstruct the original structure of the document.Within a reader, the landxml_element_id will be unique to each feature, although the writer does not rely on this uniqueness when using these attributes to determine the hierarchy of the relevant features. The writer uses the landxml_element_id and the landxml_parent_id attributes to determine containment relationships for the destination data. In cases where filtering or duplication has altered the relations determined by the landxml_element_id and the landxml_parent_id, missing parents, or duplicated element ids will result in the use of a default container.

Note that while it is possible to create cyclic containment hierarchies (i.e. with a Parcels feature referencing a Parcel feature as its parent, which in turn references that Parcels feature as its parent). These will cause the translation to fail with a warning that a cyclic relationship was detected. Self loops (in which a feature refers to itself as its parent) will be detected and the parent reference changed to the default parent in the document.

Of the feature types currently read, only Parcel/Parcels and CgPoints elements can create a cycle. A Parcels element is a container for Parcel elements, and a Parcel element can contain a Parcels element. In the case of CgPoints, A CgPoints element can contain other CgPoints elements.