TECHNICAL TIP ~ InRoads DTM Conflicts, Part 1

Okay, it’s finally time for me to lay down the foundations for the definitive word on ‘crossings’ and ‘errors’ in the InRoads Surface model. This technical illumination will discuss:

1)      An introduction to the most common type of surface problem

2)      The 3 InRoads commands / tools that identify surface ‘issues’

3)      Getting the variations in InRoads terminology finally understood

4)      How to locate Surface problems

5)      Surface ‘problems’ versus ‘alerts’ or ‘notices’

6)      Define exactly what constitutes a surface problem

7)      Begin to “scratch the Surface” on correcting problems (pun intended)

AN INTRODUCTION

Anyone who has been working with InRoads surfaces for any length of time probably already knows about the situation called ‘crossing breaklines’. It’s when two breaklines cross each other and the elevation along one of the breaklines is not the same as the elevation along the other breakline at their common intersection. This is easily illustrated as a breakline like this \ cutting across a breakline like this / and you get this X. And the elevations at the exact intersecting location are different from one breakline as compared to the other. This problem enters into the surface model because InRoads surfaces are ‘planimetric’ models where each Northing / Easting or X, Y can only have a single Elevation or “Z”. For instance you cannot have an InRoads surface model that represents a spherical shape like a basketball. Looking downward in “plan view” on the basketball, each location has 2 elevational positions – one at the top of the ball and one at the bottom. This cannot be modeled in InRoads at this time.

Beside the intersecting type problems that can occur in a surface model, another thing that often happens is duplication. Duplication refers to the instance where a specific location in a surface model is 3 dimensionally defined by more than one feature, but the X, Y, Z (or N, E, Elevation’s) are identical. This type of problem isn’t really a huge problem for the triangulation since the data doesn’t present any conflict in the model, it’s just redundancy. In order to take this type of problem one more step, let’s back up and take a general look at the surface model itself.

If anyone has been reading my books, or taken one of my fantastic classes : ) you’ve heard me continually expound upon the nature of an InRoads surface model in that there are two different aspects to a DTM – its 3D Data (features, parts, or pieces), and its Triangles (relationships, connectivity, or planar facets).

Now let’s go one step further. Taking these two basic errors, we’ll find that because of combined errors (and I use the term ‘error’ loosely) that there are really four different ‘error’ scenarios. Let’s start to break these down in more detail:

1)   Duplication, Type 1 – XYZ redundancy, both 3D features have identical elevations

2)   Duplication, Type 2 – XY redundancy, the 3D features have different elevations

3)   Intersecting Data, Derived Point – Elevational Conflict at an Intersection where the point is a derived point along the linear features and not an actual point on either feature.

1.  Elevation conflict

2.  No Elevation conflict

4)   Intersecting Data, Physical Point – Data that crosses

1.  Elevation conflict

2.  No Elevation conflict

Upcoming ZI Issue #82: InRoads DTM Conflicts, Part 2 - So what exactly constitutes a problem in the Surface?