3.3 The DATUM_PLANE entity

Datum_planes are used in conjunction with the site entity to reconstruct simple groundwater_features such as bores in 3-D space (Figure 3.2). For example, with the site defining the geographical position of a bore collar (as well as dip and azimuth for inclined bores) and a datum_plane defining the elevation of the top of the bore casing, the casing details can be spatially reconstructed.

Datum_planes are also used to provide the reference elevation for groundwater measurements. The choice of reference point is usually the one that is the most stable and convenient for conducting the measurements. Different types of datum_planes exist, such as the natural ground surface, the top of casing or the top of the kelly bushing during drilling. In subartesian bores, the depth to the standing water level is subtracted from the elevation of the measuring point (eg. top of casing) to derive the elevation of the potentiometric surface. This allows direct correlation with water levels from other groundwater_features, allowing the potentiometric contours for an aquifer to be mapped. Hence, the top of the casing is a datum_plane and requires a measurement or estimate of its elevation in metres AHD.

Table 3.3 Attributes of the datum_plane entity

A groundwater_feature can have many datum_planes. Each datum_plane for a groundwater_feature is assigned a sequential datum_ number (eg. 1,2,3..) unique to the groundwater_feature. In combination with the feature_ identifier this provides the primary key, and this is used to link data such as water levels to their appropriate origin of measurement.

The datum_plane has an elevation defined as the vertical distance in metres from sea level (0m AHD), with elevations above the origin as positive. The error attribute defines the margins above and below the recorded elevation that would effectively encompass the real elevation of the datum_plane. Hence, if the elevation error is ± 0.1 metres then 0.1 is recorded as this attribute. The most accurate estimate or measure of the elevation that is available for the datum_plane should be recorded. Hence, attributes such as the elevation, error and method values can be updated over time. For example, the elevation of the ground surface at the site may have been initially estimated from a 1:100,000 scale map, but then be improved by a subsequent survey. The datum_to_ground records the vertical distance from the datum_plane to the ground surface, and follows the convention that a positive value is used if the ground surface is below the datum_plane (refer Convention 4.2).

A datum_plane also has an operational status (refer to the status entity). It may only act as a reference for a limited time period before being superseded, such as when the original casing is removed and replaced during bore rehabilitation. In this case, an additional datum_plane is defined for the groundwater_feature, to establish the reference for subsequent data that is collected.

Details about where the datum_plane data were derived from can be recorded in attributes such as bibliographic_source, person and organisation. In particular, if the elevation of the datum_plane was estimated from a topographic map then the scale, contour_interval and contour_interval_unit can also be provided. In general, the error of an elevation interpolated from map contours is half the contour interval.

The establish_date is the date that the datum_plane was first used as a reference for data collection and is not necessarily the date that it was physically constructed or the date that the elevation was defined. This date relates more to when the datum_plane was first used to reference measurements. The measure_date is the date that the elevation of the datum_plane was measured or estimated, and may be a considerable time after the datum_plane was used as a reference for field data. In fact, a datum_plane can still be defined and function as a reference, without any estimate of its elevation.

Although the geometry of groundwater_features such as bores, wells and their inclined variants can be adequately described using the combination of a site and a datum_plane, other features cannot be spatially represented in this way. The minimum requirement for features such as trenches and drains is a series of line segments or curves in three-dimensional space. Even bores that have been surveyed and shown to deviate from their anticipated vertical or inclined trajectory may also need to be portrayed as lines or curves. Features such as seepages or effluent lakes may be treated as polygons on the land surface while caves or mines may need to be represented as proper 3-D volumes.

Initiatives have been established in recent years to establish standards for geographical information, including common protocols for how to represent natural phenomena in digital form. The Open GIS Consortium, Inc (OGC) has members from the GIS and IT industries, government agencies and academia. The International Organisation for Standardization has established a technical committee (ISOTC211) with a goal of producing standards for geographical information (ISO 15046) by 1999.

It is proposed that international standards for coordinate geometry (eg. curve, line, polygon, surface, solid) when released, be adopted for the portrayal of the groundwater_features with greater spatial complexity.