I can select a range of 3d locus only, unfortunately not the contours (poly3d) through the range of z elevations -like you want-...but, let me show you the custom selection 3d locus procedure, maybe this will be a useful procedure for you:
If you get the 3D locus of your active DTM model through the data file import, (generally a text file obtained from a data collector: Hp48GX, Sokkia SDR 31-33, etc) and this locus have the coordinates and elevations data (into the survey data record) you can select a locus elevation range:
1-Go to "organize" menu
2-Active "Custom Selection"
3-Let active the 'select' buttom
4-Pick the "create script" buttom
5-Click the criteria buttom
6-From 'List Offset whose' select "field value" "Z" and use the resources '<' and '>' to indicate the range of selection: like all z between 600m and 625m...click ok or done, and now you have a new macro selection.
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J U A N