Osnap Tools

[ypodomi]

When working inside RiverCAD commands or menus, such as Cross Section Description etc, the AutoCAD object snap selection tools are inactive. This results to inaccurate measurements, e.g. when measuring the left-right-channel lengths by picking points from the topo map, one cannot define the points using osnap tools such as “intersection – endpoint – center “ etc. Certainly any other selection is inaccurate – even the same person cannot possibly measure the same distance twice and come up with the same lengths, if not using the osnap tools. Any suggestions?

[C Westbrook]

I wouldn't think that great accuracy would be required for this input. The point and click method you described can get you to within a foot or two probably? It doesn't seem that the model would be too sensitive to this variable.

I am sure there are greater errors built into the various other inputs you have to choose, such as; the stream topography that was probably surveyed at 50 ft sections and iterpolated, the Manning's n value, the various assumptions in the method you've used to calculate your hydrographs and peak flows, etc.

[ypodomi]

Accuracy in picking points is certainly required, although is has nothing to do with model construction. I absolutely agree that there are many theoretical assumptions, both hydrologic and hydraulic, as well as errors in topography representation. Therefore nobody could possibly claim that his model is an exact copy of the real stream.
However, accuracy in picking points has to with the stream plan view being consistent to the stream profile (both of them have to be delivered to our client, with 100-meter marks on the axis). This holds for all construction drawings: if you were to deliver layout/cross sections drawings for a house, shouldn’t all dimensions on the plan view be exactly the same as the ones on the cross sections and front view? This can only be achieved using the AutoCAD osnap tools.
We are working on a 4.000 meters stream, have cut 200 cross sections. If the flow length error (due to inaccurate distance measurement inside RiverCAD) is just 0.05 meters for every cross section, the total error will add up to 10 meters. This practically means that there will be a 10 meter difference on the axis line in the plan view and in the stream profile. So, yes, great accuracy IS required inside AutoCAD!

[C Westbrook]

I see. Having to match up the stationing on the profile would be a little tough. Hopefully you'd end up close based on the chance that you'd overshoot your distance half the time and undershoot it half the time. Because the distance between sections is user defined you could tweak it slightly to get the total length to equal your plan view. But I see how you'd rather not have to "tweak it" and just be able to get it right the first time.

[ypodomi]

I will do all the draft work using the inaccurate measurements, since I believe that the model will not be affected. However, for the final drawings, I will measure all the river flow lengths accurately, i.e. using AutoCAD osnap tools, and update the data in the Cross Section Description dialog boxes manually (it's a lot of work and I'd rather do it only once). I will then re-run the program, in order to get consistent stationing both in plan view and in profiles.
Thanks, anyway!