[Manifold-l] Need some help with lines and slopes

[JBurn_GIS]

A couple of thoughts come to mind involving spatial SQL.

First, the set up:
Don't bother with the actual line at this time, just draw your the individual inflection points in the order of how your line direction will move.  So you should have a surface and a point drawing, with the points in a "from-to" order.  Now transfer your heights to those points.  You should now have points in a proper order that have Z values.  Please note that the KEY to this method is that your points are in a good logical order.

Now, assign an ID number in a sequential order to your points.  Now for the spatial SQL part.  The following is an example of some SQL that I used to perform a similar task:

select
     [point drawing].[Geom (I)],
     [point drawing].[ID] as [from],
     [point drawing].[Z value] as [from z], 
     [Copy].[ID] as [to], 
     [Copy].[Z value] as [to z], 
     (([Copy].[Z value] - [point drawing].[Z value])/(Distance([point drawing].[ID], [Copy].[ID]))) as slope, 
     ([Copy].[Z value] - [point drawing].[Z value]) as [Difference]
into [new table]
from [point drawing], [point drawing] as [Copy]
where ([point drawing].[ID] + 1) = [Copy].[ID]

Ok, running though it.  Note that I have used only one table, but selected it twice calling it "copy" the second time.  Sort of a "virtual table" if you will.  I'm linking the table and its "virtual" self with ID and ID+1.  This causes the equasions to happen between a point and the next point in order (hence the importance of getting them ordered correctly at the start).  Then I start doing some basic math and SQL selections in terms of selecting the Geom (so I can re-make my points later), my ID, Z value, slope (rise over run), and note that it is using the actual distance between the points.  In the end, you should end up with a new table (with a geom field) that contains the "from" ID and Z value, the "to" ID and Z value which is gathered from the next point in the sequence, the difference between the two, and the slope as solved by z"-z'/distance.  The results of the above query look something like this in CSV format:

Geom (I), from, from z, to, to z, slope, Difference
<geom, point>, 733419, 754.29, 733420, 754.29, 0.00, 0.00
<geom, point>, 733420, 754.29, 733421, 754.26, -0.00, -0.03
<geom, point>, 733421, 754.26, 733422, 754.18, -0.01, -0.08
<geom, point>, 733422, 754.18, 733423, 754.13, -0.01, -0.05

Since we carried the Geom(I) field, you can now easily conver this to a point drawing where each point will have the slope and difference between "it" and the next point, which could in turn be pushed on to a corresponding line segment via spatial overlay.

Hope that helps a little, or at least gives you some ideas.
Cheers.


---------------
James Burn BSc, GIS(pg), AScT
  ----- Original Message ----- 
  From: lkong at engenious.com 
  To: Manifold List Server 
  Sent: Monday, October 02, 2006 9:30 PM
  Subject: [Manifold-l] Need some help with lines and slopes


  Hi:

  I have a map with two drawings, a surface where I can run my mouse cursor
  over and it will report a z-value, and an empty drawing.  I would like to
  draw a segmented line on the empty drawing layer and get back the length
  and slope of the line based on the underlying z-values.  I can seem to
  figure out if this is a spatial or topology overlay problem.  The transfer
  heights transform only returns the height (sum, average, ... ) of the
  centroid of the line.  Any suggestions on how to approach this problem
  would be appreciated.

  Thanks
  Lenny



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