AGSi Standard / Geometry

Geometry rules and conventions

This section details rules and conventions applicable to the Geometry group of objects. It shall be read in conjunction with the relevant object reference sections.

This section shall also be read in conjunction with the General rules and conventions that apply to all parts of the AGSi schema.

File formats for agsiGeometryFromFile

The use of open standard or non-proprietary formats is recommended.

Open standard or non-proprietary formats that may be useful for geometry include:

• LandXML
• STEP, ISO 10303-21
• STL
• DXF (Autodesk, but interoperable)
• IFC

For surfaces and volumes, use of formats that provide an unambiguous representation of the surface or volume is recommended. TIN (triangular irregular network) and BREP (boundary representation) are examples of this whereas grid data (XYZ points) for surfaces may allow more than one valid interpretation of the surface.

The file formats to be used for the project should be described in the specification.

Ambiguous or incorrect geometry

AGSi does not provide any rules or guidance for the resolution of ambiguous or incorrect geometry.

Situations that may arise include:

• Unintended overlapping or intersecting volumes or surfaces
• Missing volumetric unit due to omission of a top surface (this may not be obvious if unit bottom surfaces not also defined)
• Unintended gaps in volumetric model
• Open polygons defined (start and end coordinates not the same) where the schema and/or model require a closed polygon

Interpretation or resolution of such difficulties is the responsibility of the user(s).

Volumes from surfaces (agsiGeometryVolFromSurfaces)

Recommended use

Use of explicit volumetric elements (e.g. using STEP, STL file formats) to represented solid/volumetric elements in a model is preferred.

Where this is not possible, e.g. software limitations, and only top and/or bottom surfaces are provided to form a 3D model, it is recommended that the model element (agsiModelElement object) should reference an agsiGeometryVolFromSurfaces object

Use of agsiGeometryVolFromSurfaces clarifies how the surface(s) are to be put together and interpreted to form an implied volumetric model. Rules for interpretation are provided below.

If the user(s) requires different rules for interpretation of surfaces then the agsiGeometryVolFromSurfaces object shall not be used and the model elements shall directly reference the agsiGeometryFromFile objects that define those surfaces. The project specific rules for interpretation shall be conveyed to other users via documentation, which should be referenced and included as part of the file/data set.

Mixing of agsiGeometryVolFromSurfaces and explicit volumetric elements (e.g. using STEP, STL file formats) within a model is not recommended.

Limitations

Use of surfaces that fold back on themselves (more than one Z value possible for an XY location) is not supported by this method. Explicit volumetric elements should be used in such situations.

Top and/or bottom surfaces

Definition of both top and bottom surfaces is recommended to minimise the risk of error.

Note

If only one of the top and bottom surfaces is defined, the model formed may be incorrect if a surface is inadvertently omitted. If both are defined, any anomaly arising from incomplete data should be obvious to the user viewing the model formed.

If only one of top or bottom surfaces are defined, it is the responsibility of the user(s) to ensure that the surfaces provided unambiguously describe the model.

Mixing use of top and bottom surfaces within a model is not recommended as this may give rise to ambiguity in interpretation.

Project requirements for use of the top and/or bottom surfaces should be described in the specification.

Rules for surfaces

The following rules apply to surfaces used by agsiGeometryVolFromSurfaces. These are necessary for unambiguous interpretation.

1. Surfaces used by all agsiGeometryVolFromSurfaces objects within single model shall not cross. Surfaces may be coincident.

2. If both top and bottom surfaces are defined, the top surface of an element shall lie above or be coincident with its bottom surface.

Interpretation or resolution of non-compliance with the above rules is the responsibility of the user(s).

Note

In practice, when modelling intermittent units which require parts of some surfaces to be coincident, it may be difficult to create surfaces that are exactly coincident due to the method of modelling. This may lead to small gaps or overlaps between surfaces intended to be coincident. It is recommended that users bear this in mind. One approach would be to assume that surfaces are coincident where they lie within a specified (small) distance of each other.

Interpretation

The information provided by agsiGeometryVolFromSurfaces should be interpreted as follows. It is the responsibility of the user(s) to check that the relevant software/applications are using this interpretation.

Note

It is hoped that software/applications will adopt this interpretation.

Rules for interpretation:

1. The interpretation described herein shall consider all of the surfaces defined within the collection of agsiGeometryVolFromSurfaces objects provided for a single model to construct the volumetric elements. Other surfaces that are defined by direct reference from agsiModelElement to agsiGeometryFromFile, e.g. for water tables, shall be ignored in this interpretation.

2. If only top surfaces are defined the volume formed for an element is between its top surface and the next top surface encountered below this (looking vertically down). The sides of the element are vertical extending down from the top surface to the next top surface. If no other top surface is encountered the volume extends down infinitely, but will be curtailed by the model bottom boundary, if used. This rule is illustrated in the diagram below.

   

3. If only bottom surfaces are defined the volume formed for an element is between its bottom surface and the next bottom surface encountered above this. The sides of the element are vertical extending up from the bottom surface to the next bottom surface. If no other bottom surface is encountered the volume extends up infinitely, but will be curtailed by the model top boundary, if used. When using bottom surface, it will generally be necessary to define the top boundary of the model, typically as the ground surface (terrain).

4. If both top and bottom surfaces are defined the volume formed for an element is between its top surface and the bottom surface, which shall lie below it. The sides of the element are vertical extending down from the top surface to the bottom surface. If the bottom surface is not encountered the volume extends down infinitely, but will be curtailed by the model boundary, if used. This rule is illustrated in the diagram below.

   

5. The volumetric elements shall be formed using the above rules first. If a model element includes a limiting area (areaLimitGeometryID attribute of agsiModelElement) then this limiting area is applied after the volumetric element is formed.

6. Model boundaries (using agsiModelBoundary) shall be applied after the volumetric elements have been formed and limiting areas applied.

Note

In practice, it is possible that the area extent of surfaces may vary from layer to layer, typically due to the method of modelling. This may lead to unintended consequences near the edge of the model. This may be overcome by using a model boundary (agsiModelBoundary) to clip the errant edges of the model.

Intermittent units and lenses

An intermittent unit is a volumetric unit that does not extend to the full entent of the model, i.e. it has zero thickness in some parts of the model.

A lens is a an intermittent unit that is fully or partially enclosed by another unit, i.e. enclosing unit occurs both above and below the intermittent unit.

Following the above rules, two methodologies are available in relation to intermittent units when using agsiGeometryVolFromSurfaces:

1. Surfaces for intermittent unit extend beyond the extent of unit, which may be the full extent of model, but coincident surfaces are defined to produce a volume of zero thickness in areas where the unit is not present.
2. Surfaces for intermittent unit are curtailed to the extent of the intermittent unit.

Note

If Method 2 is used, a separate model element will be required for each occurrence of the intermittent unit. Method 1 allows use of one model element for all occurrences.

The above are illustrated in the diagram below.

For a lens the method of interpretation used renders it necessary to split the enclosing unit into separate model elements representing the volumes above and below the lens. A variant of method 1 may be used as shown in the diagram below:

Note

The above solution for a lens is not ideal as it creates an artificial split in the unit. Explicit volume modelling is likely to provide a more elegant solution for lenses.

Project requirements for modelling intermittent units and lenses should be described in the specification.

Area from lines (agsiGeometryAreaFromLines)

The agsiGeometryAreaFromLines object may be used for areas defined in 2D (e.g. cross section) or 3D space.

Where the lines and areas are in 3D space, is recommended that agsiGeometryAreaFromLines objects are only used to represent areas within vertical planes.

Recommended usage, limitations and interpretation of agsiGeometryAreaFromLines shall follow the principles given above for volumes from surfaces, replacing volumes with (planar) areas and surfaces with lines.

If closed polygons are used to form areas then agsiGeometryAreaFromLines must not be used. The model element (agsiModelElement) should instead directly reference the polygon geometry which may be defined using an agsiGeometryLine object.

agsiGeometryLayer

Use of both top and bottom elevations is recommended.

The top elevation of a layer must lie above, or be coincident with the bottom layer.

If only one of top and bottom elevations is defined, the layer is interpreted to be formed as follows:

1. If only top surfaces are defined the layer for an element is formed between its top surface and the next top surface encountered below this. If no other top surface is encountered the layer extends down infinitely, but will be curtailed by the model bottom boundary, if used.

2. If only bottom surfaces are defined the layer for an element is formed between its bottom surface and the next bottom surface encountered above this. If no other bottom surface is encountered the layer extends up infinitely, but will be curtailed by the model top boundary, if used.