What does the elevation of a junction or other node really represent, in WaterCAD, WaterGEMS and HAMMER? Is it the center line of the pipe? Ground? The invert? Something else?
Currently in WaterCAD, WaterGEMS and HAMMER, there is only one "Elevation" field. The correct elevation of a node is the elevation at which the modeler wants to know the pressure. The elevations are merely used to convert the hydraulic grade line to pressure. The relationship between pressure and elevation is illustrated as follows:
Notice that an HGL of 400 ft calculated at the hydrant is independent of elevation. However, depending on which elevation the modeler entered for that node, the pressure can vary as shown. Usually modelers use ground elevation as the elevation for the node, as it is easier to obtain and typically close to where pressure would be measured from. However as long as you are consistent, you should be OK. In other words, if you use the pipe centerline elevation, use it for all nodes, and remember that the pressure calculated at that node would be the pressure at the center line of the pipe.
If you are using air valve node elements at high points, you will want to use the air valve inlet elevation (which may be roughly the same as ground elevation). See more here: Modeling Air Valves At High Points in WaterCAD or WaterGEMS
A junction node is typically placed anywhere the modeler would like to model:
1) One or more demands. Multiple customer connection points can be aggregated into the demand "collection" of a single junction (or you can use the Customer Meter elements to model the individual customer locations, and direct their demands to a nearby junction or pipe). You do not necessarily need to model the lateral connection pipe all the way out to the customer or fire hydrant (a hydrant node element has fields where you can model the lateral losses, so the hydrant only needs to be placed on the main pipeline)
2) A significant elevation change. For example in a hilly area you might want to add more junctions to ensure that you're seeing/capturing any significant elevation changes (which could be important when analyzing pressure along the pipeline) that otherwise may not have been accounted for if you modeled less junctions.
3) The intersection of multiple pipes. For example a Tee.
4) A change in pipe physical properties, such as diameter or roughness.
Elevation is especially important for a transient simulation, as the difference between positive pressure and full vacuum can be as little as 10 m or 32 ft. The formation of vapor can have a significant impact on the transient results, so accurate elevation at critical points is therefore of significant importance.
In a transient simulation, the point that the modeler needs to measure pressure from is at the pipeline elevation itself, due to the consideration of vapor pockets. In most cases, relative to the accuracy of elevation measurements/estimates, there should be little difference between the top, bottom or center of the pipe. If using a large pipe, you may consider using the center or top elevation of the pipe as the junction elevation, to be conservative.