civil3d - NAD Adjustments

[cturlington]

Hello,

I am using Civil 3D 2017.

I have been reading information about the various NAD adjustments, and that the differences between them does not constitute a change in the ellipsoid.  It sounds like the adjustments are more about the method data is collected, not about the ellipsoid.  The adjustments appear to involve more accuracy reference networks, additional base stations and networks added, not so much of changes to the ellipsoid.  So I have to ask, how do the adjustments impact the ellipsoid, if the ellipsoid does not actually change ?

[AllenJessup]

I'm by no means an expert in this. I believe that they do constitute a difference in the Ellipsoid While the NAD83 and WGS84 ellipsoids are nearly identical. They differ from NAD27. These are technically Datums built on the respective Spheroids.

Adding stations and doing readjustments to the networks, as in a HARN, would change the networks not the Ellipsoid.

Probably @Pointdump could give a much better explanation if he's around.

[cturlington]

Allen,

Thanks for your ideas on this.  One of the confusing parts to this concept is the Corpscon program has the option to select the Nad83(1986) and Harn options which would make me believe there is some difference in the ellipsoid, justifying a transformation.  But it may be that Corpscon is not performing a transformation on these but only a re-projection ?

[AllenJessup · Accepted solution]

I can understand your confusion. HARN is a "better fit" solution for that network of ground based monuments and Reference Stations. It's not a different datum. HARN is base on the same Ellipsoid as NAD83 just referencing tighter control.

In New York State we have 4 different projections, all based on NAD83, which the HARN is tied to. They use either a Transverse Mercator projection or a Lambert conformal conic projection. There are probably over 2 dozen published Datums based on them or UTM. They're all based on the WGS_1984 Ellipsoid. So different projections and datums based on the same Ellipsoid.

So it you have data which was collected using the original NAD83 system or data which is published in NAD83. Corpscon will adjust that data based on the location of the older references as located in the HARN system. Basically just tightening up the old reference network. This is a result of being able to GPS documentation that was set by terrestrial travers before there was GPS.

Hope this is little clearer than mud.

[AllenJessup]

This is a result of being able to GPS documentation momumentation. I thought I'd rejected the suggested change in spell check. Obviously missed that one.