[Manifold-l] Need some help with lines and slopes
JBurn_GIS
jburn_gis at cogeco.ca
Mon Oct 9 21:27:09 CDT 2006
>>snip>>>
I assume that this will enable me to use the same
table for multiple line segments, which means I will need to create another
field to keep track of each distinct line segment.
<<<snip<<<
Yes, while not necessary (there are ways of doing the spatial overlays etc.), this would be easiest.
Your approach sounds good. To be honest, I always just use saved selections and spatial overlays as opposed to scripting. Mainly due to my poor scripting ability;-)
Cheers.
---------------
James Burn BSc, GIS(pg), AScT
----- Original Message -----
From: Lenny Kong
To: manifold-l at lists.directionsmag.com
Sent: Saturday, October 07, 2006 4:31 PM
Subject: [Manifold-l] Need some help with lines and slopes
Thank you for your quick responses, I apologize for my slow response.
Dimitri: Definitely a technically superior approach to what I had in mind.
Perhaps a overkill.
Jim: Your approach suits me better. I was really only interested in
getting the slope and length between two points. While your solution works
(I haven't tried it yet), it requires that I define the points in sequential
order, which is reasonable, but also requires that I assign an id based on
that sequential order. I assume that this will enable me to use the same
table for multiple line segments, which means I will need to create another
field to keep track of each distinct line segment.
Is it possible to trap the insert point event? If it is possible, then can
I populate a table with the from - to information? My plan would be to
write a script that would:
1. Activate a new sequence flag and enter into a loop that would
2. capture each insert point, the first one would deactivate the new
sequence flag and identify a previous point id.
3. insert into a table a new row consisting of From, To, perhaps horizontal
distance between them.
4. return from the capture ready to capture the next event.
5. Eventually, I would need a way to drop out of the loop.
Once I know that I have a workable approach, I can confidently pursue
implementation. Although I haven't worked with Manifold before, I didn't
see anything in the documentation at sounded like capturing events.
Lenny
----------------------------------------------------------------------
Message: 1
Date: Tue, 3 Oct 2006 16:41:38 -0700
From: "Dimitri Rotow" <dar at manifold.net>
Subject: RE: [Manifold-l] Need some help with lines and slopes
To: "'Manifold List Server'" <manifold-l at lists.directionsmag.com>
Message-ID: <200610032341.k93NfUZ2029019 at web2.directionsmag.com>
Content-Type: text/plain; charset="us-ascii"
> 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.
You'll have to script either of these. The two tasks are related as they
are both riffs on computing elementary values of a right triangle.
First, though, let's consider the "slope" of the line. A line traversing an
irregular surface will have no single slope value. Consider, for example, a
road that goes up a hill and then down a hill. There's no one "slope" to
that road. There may be a slope for any one straight line segment, and you
might compute an "average" slope by an arithmetic aggregation of slopes for
individual line segments that comprise the polyline, but if you want to
attach just one "slope" number to the line you are going to have to decide
what sort of approximation or aggregation you want that number to be.
To get the 3D length of a line you first decompose it into individual line
segments. You know the height of both ends of each segment, so you know the
difference in height between the segments. You know the length of the line
segment. Using the Pythagorean formula you can find the length of the
hypoteneuse given the length of the base of a right-angle triangle (the
length of the line segment) and the height of the triangle (the difference
in z of the end points). Add the hypoteneuse lengths of all the line
segments for the line to get the length of the line.
You could use a similar process to compute the slope of each segment. You
know the length of the segment and the difference in heights and even (from
the above) the length of the hypotenuse. It's therefore a simple
calculation to get the value of the acute angle between the base and the
hypoteneuse. Once you have the slope for each line segment you can decide
how you want to aggregate them to get some sort of average or combined slope
for the line.
You could simplify the script by doing some manual work. For example,
create points at each coordinate of a line and then transfer heights from
the surface to each point. This and suitable "flag" attributes used wisely
will simplify the scripts.
Cheers,
Dimitri
------------------------------
Message: 2
Date: Tue, 3 Oct 2006 23:20:32 -0400
From: "JBurn_GIS" <jburn_gis at cogeco.ca>
Subject: Re: [Manifold-l] Need some help with lines and slopes
To: <lkong at engenious.com>, "Manifold List Server"
<manifold-l at lists.directionsmag.com>
Message-ID: <000801c6e764$0d415890$6700a8c0 at Chimera>
Content-Type: text/plain; charset="iso-8859-1"
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
_______________________________________________
Message: 3
Date: Tue, 3 Oct 2006 23:26:06 -0400
From: "JBurn_GIS" <jburn_gis at cogeco.ca>
Subject: Re: [Manifold-l] Need some help with lines and slopes
To: "Dimitri Rotow" <dar at manifold.net>, "'Manifold List Server'"
<manifold-l at lists.directionsmag.com>
Message-ID: <000f01c6e764$d44511c0$6700a8c0 at Chimera>
Content-Type: text/plain; charset="iso-8859-1"
Dimitri makes a good point (as usual;-) in that the method I described give
the slope between points, and not necessarily of the surface. Sort of a
slope "how the worm digs" so to speak. Technically, if you added more
points, so you have "lots" of points traversing the length of the line,
these could then be combined or looked at separately.
For one job, for example, I have a bunch of points that all line up for
about 200 meters. Looking at the slope values though, one can see that the
slope actually goes from a 5% grade, to a 10% grade to a 7% grade over the
200m distance. For that particular task, I grouped the points by rounded
grade, and created the line segments appropriately, so instead of one 200m
line at an averaged (ok, average isn't the right word) grade, I had three at
their individual slopes.
Cheers.
---------------
James Burn BSc, GIS(pg), AScT
----- Original Message -----
From: Dimitri Rotow
To: 'Manifold List Server'
Sent: Tuesday, October 03, 2006 7:41 PM
Subject: RE: [Manifold-l] Need some help with lines and slopes
> 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.
You'll have to script either of these. The two tasks are related as they
are both riffs on computing elementary values of a right triangle.
First, though, let's consider the "slope" of the line. A line traversing
an
irregular surface will have no single slope value. Consider, for example,
a
road that goes up a hill and then down a hill. There's no one "slope" to
that road. There may be a slope for any one straight line segment, and you
might compute an "average" slope by an arithmetic aggregation of slopes
for
individual line segments that comprise the polyline, but if you want to
attach just one "slope" number to the line you are going to have to decide
what sort of approximation or aggregation you want that number to be.
To get the 3D length of a line you first decompose it into individual line
segments. You know the height of both ends of each segment, so you know
the
difference in height between the segments. You know the length of the
line
segment. Using the Pythagorean formula you can find the length of the
hypoteneuse given the length of the base of a right-angle triangle (the
length of the line segment) and the height of the triangle (the difference
in z of the end points). Add the hypoteneuse lengths of all the line
segments for the line to get the length of the line.
You could use a similar process to compute the slope of each segment. You
know the length of the segment and the difference in heights and even
(from
the above) the length of the hypotenuse. It's therefore a simple
calculation to get the value of the acute angle between the base and the
hypoteneuse. Once you have the slope for each line segment you can decide
how you want to aggregate them to get some sort of average or combined
slope
for the line.
You could simplify the script by doing some manual work. For example,
create points at each coordinate of a line and then transfer heights from
the surface to each point. This and suitable "flag" attributes used wisely
will simplify the scripts.
Cheers,
Dimitri
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