Being Smart with Z Values

Owned by Steve Peregrine
Last updated: Dec 05, 2020
3 min read
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Chapters

Understanding Z Values

Z Values 1

Understanding Z values.

The HADDMS database asset management system does not store coordinate shapes in 3 dimensions, only 2, but the area clients usually insist on 3 dimensional coordinate data in their shapefiles so that they can use the data for design and modelling, and they also usually demand a higher level of GPS accuracy than is required by the HADDMS database.

The usual measurements that are made by the crews on are site are:

  • The GIS crew will measure the X (Easting), Y (Northing) and Z (Altitude above Sea Level) values of the centre of the point asset cover with GPS equipment, known as the Cover Level (CL), point shapefile field COVER_LEVE.

  • The CCTV crew will measure the depth of the ends of the pipe below the cover level with a tape measure, known as the Invert Depth (ID), continuous shapefile fields UPSTR_DEPT and DNSTR_DEPT.

The term ‘Invert Level’ (IL) is misused by many drainage contractors to when they are referring to the measurement down from the cover to the invert of the pipe. As described, this measurement is the ID of the end of the pipe, and the IL is height above sea level of the end of the pipe, and this cannot be measured with a tape measure.

From these values, we can work out:

  • IL = CL - ID

In WinCan VX, there is a button in the Tools ribbon called ‘Calculate Invert Levels’ which when clicked will calculate the IL for all pipes and nodes where the two required data fields have valid values.

This is all good and well when the GIS crew and the CCTV crew have collected good data on site and where the node at each end of the pipe is a drainage asset like a manhole, gully or catchpit.

But what do we do when the downstream node is a connector node (or any other type of modelling node)? In this case, there is no cover to measure the altitude of, so how can we ensure good data and auto-calculations from the tool in the ribbon bar?

To to ensure good data, assuming you are inspecting a lateral gully leg that goes down to a connector node on a main section:

  • Do not enter a value for the depth at the downstream end of the pipe. Do not try to guess it. Just leave it blank or set it to zero.

Warning - only do this where the point item at the end of the continuous item is a Connector Node. Continue to set the other end of the pipe depth in the normal way.

When the connector node is plotted on the pipe, it’s altitude will be calculated from the geometry of the section that it is connected to, so its Z value in the node header will be exactly where it is, at a point under the ground.

Keeping the pipe depth blank or zero means that this value will not be recalculated when the ‘Calculate Invert Levels’ button is clicked so the 3D geometry of the network will remain in tact.

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