Splines - really offsetting!

I have a spline with many nodes - on the scale of the free-form horseshoe bends of a stretch of river - typical radius about 25m. I want to offset 15no 4m parallels both sides of that, including the weird reversals and overlaps where the parallels collide on the inside of a bend.

First problem - Acad often refuses to offset a spline, but if the spline is converted to a very-multipoint polyline e.g. with Flatten, then it will offset.

Second - though Acad then offsets, it leaves out all the weird bits (as above) that I do want. I tried Explode, then offset the segments individually, but the segments are still curved, so offset still doesn't happen once the offset line would go 'behind' the arc centrepoint.

I could draw a large number of straight lines along the curve, each of which would offset independently and could be chamfered/trimmed/extended to its neighbour.

Any easier ideas?

I have not doen this complex use in past, but I remember once having trouble & I could manage it by breaking(trimming) the splines at those odd turns & than ofsetting & filling(adding) those bits in.

When you have changed your spline into a polyline, use PEDIT to decurve, and then offset. Then recurve all the polylines. Not perfect, and you will probably have to do some editting (i.e. inserting extra nodes) to give the shape you want.

Splines are difficult to manipulate, and I always use polylines in mapping because of ease of editting

Thanks eldon - unfortunately offsetting the decurved polyline still leaves out the interesting bits of the offset line, where it 'goes behind itself' on the inside of a bend.

dipali, thanks - could you explain more - seems I'd still have curves, which would still refuse to be offset 'behind their centrepoint'.

Attached is doing it by exploding into individual straights, offsetting all by one 4000mm offset, chamfering together, then repeating - lot of work and already begining to get it wrong I think - 4 offsets done, 11 to go! It's the curve reversals I'm interested in - you can see them begining to happen - but not smooth enough, and a non-starter by this method. Any better ideas?

If no chance, then anyone know of a program that would do such offset a spline (or polyline) 'pure', without tidying it up in this way?

Drawing2.dwg

For what it’s worth, this is how Rhinoceros3D would create the offsets if the lines are converted back to a spline. The majority of the curve that backtracks onto itself is automatically eliminated (I'm not sure if you would consider that a good thing or not).

Offsets.dwg

Thanks SEANT - that's exactly the effect that Acad gives by offsetting the spline (when that occasionally works) or after Flattening to polyline. It's the eliminated bit that interests me - so not Rhino, unless it's got a switch to toggle this offsetting behaviour?

why are you trying to offset this? are you trying to get the beds of the river?

only thing i can suggest is copy selection and paste it you desired distance. Then also offset your orig. line and edit both so they have the feature and scale you want. I dont have acad on this computer so i couldnt open your file, sorry:(