Airport Boundaries and Global Scenery

Revision History:
	 06/25/08   Updated With Taxiline Guidelines
	 10/25/07   Initial Draft

Correct Formation of T Junctions

A "T" junction is any connection between lines in an airport layout where one line ends in the middle of another line. T junctions would include a T-shaped meeting of taxiway lines (shown in these examples), but the junctions formed by the edges of taxiway pavement are also T junctions and need to be formed using the same technique.

The key points are:

  • Two lines in a WED layout (of any type) should always meet at their endpoints.
  • One line should not end in the middle of another line.
  • This means that you must split a line at the point another line intersects it.

You should not simply attempt to visually line up your layout. Consider this T junction:

It looks correct but upon zooming it becomes clear that the taxiway line ends "near" the next line but is not actually on it. With no vertex to join to, this join can never be clean.

Junctions like this can suffer from all sorts of problems in X-Plane:

  • The alignment error may look significantly worse in the sim - the only way to guarantee a good connection is to have two vertices with the exact same location.
  • If one of the lines is a bezier curve, the approximation of the bezier in X-Plane may be different from WED, causing a disconnect.
  • If these lines were taxiway edges, the "sliver" of a gap could cause bumps when a plane rolls over it or ugly rendering errors in the pavement.

The correct way to build a T junction is a two-step process.

  1. Split one of the lines so there is a vertex at the location where the two nodes will meet.

  2. Then drag the connecting line to that vertex. In WED you can use the "snap to vertex" option to get precise positioning of the two vertices on top of each other.

This T junction will look correct at any zoom, and will render correctly in X-Plane.

Remember: close visual alignment in WED is not good enough! You must create an extra vertex and snap the vertices together to create precise aligned layouts.

Correct Bezier T Junctions

When a T junction is made up of a bezier curve, another issue comes into play: besides needing the end vertices to be snapped together, the bezier curve handles must not be "over-curved". Consider this layout:

It looks okay in WED, but note that the lower control handle of the bezier curve is on the left side of the straight taxiway line (and yet the bezier curve itself is on the right side. This is a mistake in the layout! Here's what it looks like close up:

The problem is that the bezier curve control handle (by being on the left side of the taxiway line) pulls the bezier to left a little bit too much. The result is that the bezier crosses over the centerline before joining with it. This error looks small in WED, but in X-Plane, where the taxiway lines aren't as smooth, the error is significantly more noticeable.

In order to fix this problem, the lower bezier control handle must be on the same side of the joining line as the curve itself - in this case on the right side.

This creates a bezier curve that looks good no matter how closely it is zoomed.

The key here is to err on the side of caution: position the bezier control point on the right side of the connecting line; do not trust the preview of the curve to show if there is a problem.

Boundary Polygon in apt.dat and Global Scenery

When we make the global scenery, we apply special processing to the surface area of airports, as defined by their boundary.

  • The elevation of the airport area is pre-processed to remove bumps and radar spikes (flattened)*.
  • The terrain type is set to airport grass.

Before the apt.dat 850 file format, the airport boundary was calculated by taking all of the airport pavement and adding a buffer region. But with the apt.dat 850 file it is possible for an author to draw the airport boundary by hand.

Guidelines for drawing the airport boundary:

  • The airport boundary should match a visual boundary between cleared airport area and surrounding terrain.
  • The airport boundary should include any airport-related buildings, so that the elevation near terminals is well-controlled.
  • The airport boundary does not need to be very detailed - the boundary is slightly blended in the DSF file.

This picture shows a possible boundary for BWI. Note that the forest areas are probably on the property of the airport legally, but are not part of the airport boundary in the diagram so that the trees are not replaced with grass when the airport is built.

If you make an X-Plane 850 airport layout, you should create a boundary polygon to indicate where the global scenery should stop and the airport should start.

Do Not Add Extra Vertices To Make Smooth Curves

You must add vertices to make T junctions correct. Howeverm you should use the smallest number of vertices possible to correctly describe your layout. For example: this is a bad taxiway line.

The taxiway line is bad because it uses a huge number of vertices - way more than necessary. The line should be built like this:

Use a small number of bezier curves to shape your lines. Do not add extra vertices in an attempt to make the line look smoother in X-Plane.

X-Plane uses an "error" heuristic to draw taxiway lines - basically it will add vertices to minimize the "squareness" of lines. The problem with adding vertices is that you have to add a very large number of vertices to force X-Plane to render at a higher quality. If you add a few vertices, you increase file size, download time, load time, and RAM use, and X-Plane simply adds fewer of its own points (since fewer are needed to reach the same amount of roundness). Only by adding an insane number of vertices can you markedly improve visual quality; this comes at the expense of RAM use for your layout.

Leave smoothness of lines up to X-Plane; this setting will be wired to the user interface. Some users will need lower rendering quality to use your airport on slower computers. If you add too many vertices, you simply assure that these users cannot fly to your airport at all.

Always Check Log.txt

Before submitting your data to Robin (see below) or publishing your custom scenery, always load your scenery, quit, then check X-Plane's "Log.txt" file (in the same folder as X-Plane itself) for error messages. Some problems with your scenery may only be logged to the error file, or may be described in more detail in Log.txt. Do not ship your scenery until you have addressed all log.txt problems that come from your content.

Submitting layouts to Robin Peel

Robin Peel manages the X-Plane airport database. You can email him new layout data. Some guidelines:

  • The sooner you submit your layout, the more likely it is that the airport area will be flattened in the global scenery.
  • If your layout is not done but you would still like it to be flattened, tell him you are submitting the layout only for global scenery creation - we'll use it to clear the DSF and you can finish the airport later.
  • If you need to submit a partially finished layout, make sure the airport boundary is correct and follows the rules of the apt.dat file format.