1. there are Location 2D objects below the hierarchy,
2. the Location 2D objects are in the scope of a 3D spatial reference frame, and
3. you (the data provider) want the locations to lie on a surface other than the last default surface (The initial default is the spatial reference frame's vertical datum).

The solution is to mandate a clearly defined surface (the initial default) and provide a flexible mechanism to override the default for all or for selected parts of the transmittal.

1. Normal to the surface of the SRF ellipsoid*,
2. Intersects the ellipsoid at the same horizontal coordinates as the Location 2D, and
3. Extends below the surface of the ellipsoid to a depth equal to the minor radius of the ellipsoid.

(* NOTE:
For augmented Projected SRFs, this is the projection ellipsoid. For LSR, use the z=0 plane, where z is the coordinate axis specified by the SE_LSR_3D_PARAMETERS up_direction value. )

The intersection of this ray with the resolution surface defines the candidate set for the corresponding 3D location. One 3D location is selected from the ray/surface intersection set according to the following three cases:

1. If the set contains 1 and only 1 element, the spatial position of that point resolves the Location 2D instance.
2. If the set contains more than one element, the Reference Surface field multiplicity_rule value is used to select exactly one element. (For instance, if several overlapping Property Grids with Grid Overlap components are part of the Reference Surface, use Grid Overlap's rules to define the Reference Surface in the overlap region of two or more of the surface grids. If the intersection set still has more than one point, use multiplicity_rule.)
3. If the intersection is empty, then look for other Reference Surface objects higher up the aggregation tree and repeat this resolution process with that surface instead. If there are no other Reference Surface objects higher up the aggregation tree, then use the vertical datum, which is guaranteed to be a case 1 surface. (See also example 5).

1. The Geometry Hierarchy is a Property Grid Hook Point that aggregates at least one Property Grid with these qualifications:
   
   1. its data_table_type field is equal to the Reference Surface's classification field,
   2. it has 2 spatial axes corresponding to the horizontal coordinates of the SRF, and
   3. it has a Table Property Description for height, elevation, or bathymetry.

   If the Property Grid meets the above criteria, then it defines a resolution surface.

2. The Geometry Hierarchy is a Union of Primitive Geometry that aggregates Surface Geometry with Classification Data matching the Reference Surface's classification field. In this case, all such Surface Geometry objects combine to form the resolution surface. (Note that the multiplicity_rule field deals with surface complexity).

3. The Geometry Hierarchy is a Distance, Index, or Map Scale Level of Detail Related Geometry that aggregates (directly or indirectly) Geometry Hierarchy cases 1 and/or 2 above under an LOD branch selected by the Reference Surface lod_rule field.

4. The Geometry Hierarchy aggregates some combination of cases 1, 2, or 3.

In general, the set of all Surface Geometry and Property Grids under the Geometry Hierarchy is culled by matching the Reference Surface classification field (and Property Grid qualifications) and matching LOD branches to the lod_rule field. The remaining geometry is the resolution surface used for ray intersections.

• If Location 2D objects are supported in the scoping SRF, (e.g. the scoping SRF is 3D geodetic, for which we have 2D geodetic), these LSR Location 2D instances are converted to Location 2D objects in the instancing 3D SRF using a 3 step process:
  

  Step 1) An LSR Location 2D is converted to LSR Location 3D by resolving to the LSR vertical datum (z=0 plane, where z is the coordinate axis specified by the SE_LSR_PARAMETERS value up_direction).
  Step 2) The resulting LSR Location 3D is converted to a Location 3D in the scoping 3D SRF in the usual (model instance) way.
  Step 3) If the SRF supports Location 2D objects (e.g. geodetic), then the Location 3D is collapsed to a Location 2D with the same horizontal coordinates.
  

• Otherwise, if the scoping SRF does not support Location 2Ds, then the LSR Location 2D is converted to 3D by setting the tertiary axis value to zero.

Case 1 - both SRFs have the same horizontal datum.
Case 2 - The two SRFs have different horizontal datums.

In case 1 (Same horizontal datum), the ray determined by the Location 2D is invariant, so the horizontal coordinates are converted in the usual way.
In case 2 (Different horizontal datums), the ray may change, so three steps are needed:

Step 1) Resolve elevation in the originating SRF and convert the Location 2D to Location 3D.
Step 2) Convert the Location 3D to the second SRF (conversion not currently supported).
Step 3) Collapse the Location 3D to a Location 2D.

Consider a Location 2D whose ray intersects the resolution surface near the center of exactly one triangular Polygon. The intersection point determines the elevation, and therefore a corresponding Location 3D. If you resolve in the first SRF, and then convert the Location 3D, the location will match the transmittal point, but it may no longer lie on the triangle's surface. On the other hand, if you convert the Location 2D and then resolve in the second SRF, the corresponding Location 3D will lie on the triangle's surface, but may differ a little in elevation from the originating point. Therefore if absolute location is more important than conformality, then resolve in the originating SRF. If conformality is more important, then resolve in the consuming SRF.