Trial Pit and Trench Features: Concepts and proposed geometry properties for 2D relative eferencing for trench walls and 2D surfaces

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Trial Pit

Both DIGGS and AGS have the concept of a trial pit, defined broadly as a relatively shallow excavation into the earth's surface, dug either manually or by a mechanical excavator. In both DIGGS and AGS, the locations of observations and samples collected from a trial pit are referenced in 1D only - eg. with reference to depth below the ground surface. There is no specification as to the absolute location of an observation or sample laterally along a wall of a trial pit. In this sense, a trial pit's geometry is modeled identically to a borehole and requires a referencePoint property (a Point object) and a centerline or trajectory property (a LineString) to define the trial pit geometry, along with an optional property to define the linear referencing system for the trial pit (allowing locations to be reported in depth only along the centerline as opposed to an absolute coordinate location). In the case of a trial pit, the centerline will always be a single line segment (consisting of two vertices) and oriented vertically (eg. the x and y coordinates of both linestring vertices are the same).

A trial pit will share some properties of a borehole, such as construction date/time, construction method, construction events and backfill information, but would also contain properties different from a borehole like those that define the overall shape of the trial pit (eg. pit depth, pit width, pit length), plus shoring information and possible other properties.

Trench

A trench is essentially the same kind of feature as a trial pit, but typically is more elongate than a trial pit in shape and usually is constructed using a mechanical excavator (backhoe) rather than by hand. In most cases a trench in plan view would be represented by rectangular polygon with walls approximating planar surfaces, although is it possible for trenches to be curved or "kinked" such that the trench walls may consist of adjoining planar surfaces. Trench walls may be vertical or sloped

Observations made and samples collected from trenches are made on the trench walls or the trench floor and are at present typically recorded as graphical drawings (trench logs) to illustrate observed features and sample locations. It is not common practice for these log locations to be represented as digital objects in 3D space, although we should be ready to accommodate this sort of representation in the future.

A trench will (should) contain the same metadata properties as listed above for a trial pit. However, the geometries of a trench are more complex, insofar as a trench consists of 4 walls (2 side and 2 end walls) and a floor), any or all of which may be logged. As each wall of a trench has its own unique geometry, we need to identify each trench wall as a distinctive feature upon which observations are made or samples collected.

Trench Wall

DIGGS v. 2.5 introduces the concept of a TrenchWall sampling feature whose geometry is represented by a planar surface. DOGGS v 2.6 (development version) extends this concept to allow a trenchwall to be represented as a multi-planar surface, consisting of adjoining planes, thus allowing for curved or kinked trench walls. Below are the geometry properties and relative referencing properties defined by DIGGS for trench walls:

• referencePoint: a point (Point geometry object) that represents the origin location of the TrenchWall described in a well-known 2D or 3D geographic or projected coordinate reference system (CRS)

• referenceEdge: a line (Linestring geometry object), whose vertices are described in the same CRS as the referencePoint, that runs through the reference point along one edge of the TrenchWall's surface. It is used to define the x-axis of the TrenchWall's 2D vector linear spatial reference system (SRS).

• featureExtent: a region that defines the limits of the TrenchWall. featureExtent is defined by either a Polygon geometry object or a Surface geometry object. A Surface consists of two or more planar surface patches (PolygonPatch geometry objects) that adjoin each other to form a continuous surface. Coordinates of the vertices that form the geometry objects of the featureExtent are described in a well-known 2D or 3D geographic or projected CRS.

• relativeFeatureBoundary: this property serves the same purpose as featureExtent except that the coordinates of the surface vertices are described by the TrenchWall’s 2D SRS, if one is defined.

It is common when reporting the locations on a trench wall using a relative cartesian coordinate system with two coordinates - 1) the distance along the x-axis from an origin point, and 2) a depth or distance along the vertical or y-axis, rather than using absolute geographic or projected coordinates. In order to relate the relative coordinates to geographic ones, we need to then define a vector linear referencing system, which is an extension of GML 3.3's linear referencing approach proposed for boreholes.
The VectorLinearSpatialReferenceSystem object derives from glr:LinearSRS (GML v.3.3) and contains the following mandatory properties:
• gml:Identifier: an identifier for this SRS.

• linearElement: a pointer to the linear geometry object that in this context would be the x-coordinate axis. This property would therefore point to the Linestring object of the TrenchWall’s referenceEdge property.

• lrm: the method used to measure along the linear element. This is defined by a GML 3.3 object with three mandatory properties: 1) name, 2) type and 3) units. In the context we are using linear referencing in DIGGS, the values for name and type are typically “chainage” and “absolute” respectively, and the units property defines the length unit of measure, typically m (for meters) or ft (for ft/US).

• offsetVector: a unit vector that defines the orientation of the TrenchWall in space and also defines the direction of the y-coordinate axis. The offset vector extends from the TrenchWall’s reference edge and is defined in 3D space, with each vector component defining the x, y, and z orientations of the SRS. The following show examples of how offsetVector would be encoded given various TrenchWall orientations:

<offsetVector>1 0 0</offsetVector> <!—Horizontal surface, positive east -->
<offsetVector>0 -1 0</offsetVector> <!— Horizontal surface, positive south -->
<offsetVector>0 0 1</offsetVector> <!— Vertical surface, positive up -->
<offsetVector>0 0 -1</offsetVector> <!— Vertical surface, positive down -->
<offsetVector>0 -0.7071 -0.7071</offsetVector> <!— 45 deg S dipping surface, positive down -->

These concepts and properties are illustrated below. This example is of a multi-planar vertical cross section with resistivity data mapped onto it, but the concept is identical for a trench wall. Beside trench wall surfaces, this geometric construct can be used for any kind of cross-sectional data, map locations - any type of sampling feature whose geometry can be modeled as a 2D surface with a Cartesian coordinate system for linear referencing.

An example of a multi-planar TrenchWall feature as a vertical surface, showing a graphical representation of resistivity within the sampled space. The feature extent of the TrenchWall bounds the data and is composed of five polygon patches (white dashed lines) that bound the planar surfaces that make up the entire feature extent. Also shown are properties that define the vector linear spatial reference system for the TrenchWall: 1) the reference point, which defines origin of the feature in geographic space, 2) the reference edge, a line that provides one boundary for the feature and is the x-direction axis; here it lies along the feature’s top edge, and 3) the offset vector, a unit vector that defines the direction of the y axis and also fixes the orientation of the surface in space. The location of any point on the TrenchWall can then be described by the coordinates (r,v) where r is the distance along the reference edge from the origin, and v is the distance along the vector orientation. Black circle shows the location on the TrenchWall at coordinate (r1,v1).