Dynamic Segmentation Help!

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12-01-2013 03:59 PM
ThiPham
New Contributor
Hi Everyone,
This is my first time performing dynamic segmentation. I was able to create the route and perform the dynamic segmentation on my route, but when I try to use the locate features along routes tool to be able to identify the measurement of over 3000 points along the route, the output measurements for the location points does not appear correct. The latitude and longitude of the location points have been changed in the output attribute table, which may be the reason why the measurements are incorrect. Does anyone know why the latitude and longitude changed in the output attribute table after I used the "Locate features along routes" tool?

Thank you for your help. Any suggestions on how to fix this problem is greatly appreciated.
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3 Replies
RichardFairhurst
MVP Honored Contributor
Hi Everyone,
This is my first time performing dynamic segmentation. I was able to create the route and perform the dynamic segmentation on my route, but when I try to use the locate features along routes tool to be able to identify the measurement of over 3000 points along the route, the output measurements for the location points does not appear correct. The latitude and longitude of the location points have been changed in the output attribute table, which may be the reason why the measurements are incorrect. Does anyone know why the latitude and longitude changed in the output attribute table after I used the "Locate features along routes" tool?

Thank you for your help. Any suggestions on how to fix this problem is greatly appreciated.


Did you recalculate those attributes after locating the points along the route?  Or are you saying that the operation of the tool itself did that?  As far as I know the tool itself should not directly alter those attributes, but it will alter the actual location of the point if those attributes are recalculated based on the event layer position using the geometry calculator.  If you are saying the tool directly altered those attributes without using the geometry calculator I would need to know more about the inputs and tool settings you used.  Are you sure the routes and points are in the same projection?

Even if the routes and points are in the same projection, the geometry calculator can change the long and lat if it is recalculated based on the event point location.  The actual location will change relative to the original point location in two possible ways in the event layer.  If no side offset is used the point will be relocated to the centerline of the route, which will change the long and lat of any point that was originally offset on one side of the line or the other.  That can be corrected partially by including a side offset distance for the point event layer.

The second way it will change the point position even if you do use a side offset is that any points that fall beyond the end of the line will project as exactly perpendicular events to the route from the end point of the line, not along the original angle from the end of the line.  So it is impossible to use linear referenced events to recreate exact locations around a cul-de-sac bulb, for example.  Those point locations will always have altered long and lat positions relative to the original point in its projected event form if they are recalculated using the geometry calculator.  Nothing can fix that other than to retain the original coordinates in attribute fields.

Another concern is that if you kept only the closest location, you will get associations to the wrong route for several of your points if they fall closer to a cross-street (or branch) than it does to the correct route.  This occurs as points near an intersecting road/branch.  I always uncheck that option and use an attribute to make sure I am associated to the correct route, throwing out false route links.

Other possible concerns would arise if your routes are not simple polylines with continuously increasing measures.  Branches will cause problems if you included them in your routes.

Points which are exactly perpendicular to a simple route that use the correct side offset distance in the event layer should have measures that result in the same long and lat values as the original point if the projection was not changed.

So explain what you did a little more clearly.  I have never seen the effect you seem to be describing if you did not use the geometry calculator to overwrite your long and lat values.
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ThiPham
New Contributor
Hi rfairhur24,

Thank you for your help. The route that I created is a line polyline that has measurements along the route, not a network of intersecting routes.

What I did was that I selected the polyline I wanted to create a route and used the Create Route tool. Then, I projected both the polyline and the points feature from GCS North America 1983 into a projected coordinate system, NAD 1983 UTM 10N. Afterward, I created a database event table using the excel table I created for the measurements along the route. After I created the route, I used the Make Route Event Layer tool to perform the dynamic segmentation on the route I needed. Finally, I used the Locating Feature Along a Route to locate the points relative to the dynamic segmented route. After performing a quality check on the output attribute table created by the locating features along a route tool, I realized that the latitude and longitude has been changed from the original point feature attribute and the measurements along the route was also incorrect.

One of the problems I had while performing the dynamic segmentation was not being able to flip the dynamic segmentation starting point to the opposite end of the route. Therefore, I relabel the description of the measurements to display the right measurements, which is basically just flipping the measurements. I do not think this is the cause for the modified latitude and longitude, but it may be another cause for the incorrect measurement.

Can you elaborate on this statement: "As far as I know the tool itself should not directly alter those attributes, but it will alter the actual location of the point if those attributes are recalculated based on the event layer position using the geometry calculator."?

Thank you for your help.

-gisjoy
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RichardFairhurst
MVP Honored Contributor
My prior post was the elaboration on what I meant by that sentence.  Certain point locations along a line cannot be recreated by an LR event, such as points that curve around the end of a cul-de-sac bulb beyond the end of the route.  Only points which can be offset perpendicular to the route at the measure location can be exactly placed by an LR event table.

Choosing one route does not mean it is a simple route without branches if multiple original polylines created it or the one polyline that created it was self intersecting.  A single route can branch, but when it does the measures get messed up as far as every measure being a unique position along the line.  Also, the Create Route tool is not perfect if it creates a route from multiple polylines that do not meet end to end and that overshoot each other when they actually should touch.  Bad topology like that must be eliminated before using that tool.

The Coordinate priority of the Create Route tool should let you choose the orientation that the route will be built.  If you have preassigned measure fields in your table they can also control the way that tool builds the route.  So I am not sure why you needed to flip the route after building it.  If you have built it from the upper left corner flipping the priority to one of the other corners should alter the route creation orientation.

I would convert the excel table to an actual point feature class by exporting the XY Event layer first before running the Locate Features Along Route.  I never use XY Event tables to create Route Event tables directly.  An actual point feature class should not alter the long or lat values when the events are located along the route.  I have seen XY Event tables alter the coordinates in the long and lat fields when python cursors operate on them, but never on a real point feature class.  Whatever is happening to the long and lat fields, the Linear Referenced events layer does not use the Long or Lat fields when it is displaying the LR event along the route, only the RouteID and the measure (and optionally an offset distance field).

A picture is worth a thousand words.  An example screen shot showing an original point location and a linear referenced event that does not match the original position may explain what is really going on.  If you can show the hatched measures of the Route that should also help.
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