Extrusion Bridge idiosyncracies

[Bungo]

Is there a formula for the altitude of extrusion bridges that I'm missing?

I have the following code for an extrusion bridge, the altitude figures for the start and end were eventually worked out by trial and error (read: modify/compile/reload/observe/guest-imate correction/repeat) but it looks nothing like what I measured both in-sim and in Global Mapper with the Terrain mesh for the ends of the bridge. And at the reload section it takes a fair amount of time.

The end points are on a 1m Mesh based on LIDAR data, and other than altitude match perfectly with the terrain in the sim. The Altitude Sample Locations are the first positions in from either end and are both in the Derwent river at 0 elevation (sea level).So in theory a line connecting the 2 sample points would be 0 elevation and the points on the bridge should be real altitudes and the reference line would be conveniently horizontal. As you can see the real/measured altitudes are in blue after each point's definition, and the ones that work in-sim are in black in the XML. There isn't even a consistent offset value between the measured and 'divined' values.

The intermediate values are simply calculated by adding the "(total altitude difference between the ends / num segments) + previous point's altitude" to get a straight deck bridge at a slight incline to the horizontal.

I read Luis' tutorial, and Arno's short blog on extrusion bridge offsets here, so I knew to expect some weirdness, but If I can't quantify it, doing 20 or 30 of them by trial and error will be a nightmare.

The aircraft in slew mode in the river says it is at 6.3 feet altitude, but it also says that in the open ocean, so that might just be the distance of the ARP from MSL in the model. Even if I correct for that, the numbers are still nowhere near matching.

Does anyone have an insight that might allow me to make sense of this that I can apply to future bridges?

<!-- Bowen Bridge, Hobart, Tasmania, Australia -->
<ExtrusionBridge
instanceId="{22E1D3DC-CD97-4A5E-9C07-60F276659CE1}"
probability="0.14"
suppressPlatform="FALSE"
imageComplexity="VERY_SPARSE"
roadWidth="18.0"
extrusionProfile="{3851ace9-3b9c-4667-b8e5-dab788668415}"
materialSet="{7ed5603a-d17a-4246-bc3d-baadea6909fa}" >
<AltitudeSampleLocationList>
<AltitudeSampleLocation lat="-42.8077225305979" lon="147.304183490551"/>
<AltitudeSampleLocation lat="-42.8148146864925" lon="147.310341658501"/>
</AltitudeSampleLocationList>
<PolylinePointList>
<PolylinePoint latitude="-42.81392517" longitude="147.31142043" altitude="-3M"/> 14.40M
<PolylinePoint latitude="-42.81488103" longitude="147.31026676" altitude="1.2M"/> 15.29M
<PolylinePoint latitude="-42.81564244" longitude="147.30938702" altitude="4.9M"/> 16.18M
<PolylinePoint latitude="-42.81640385" longitude="147.30850728" altitude="8.6M"/> 17.07M
<PolylinePoint latitude="-42.81716526" longitude="147.30762755" altitude="12.3M"/> 17.96M
<PolylinePoint latitude="-42.81792667" longitude="147.30674781" altitude="16.0M"/> 18.84M
<PolylinePoint latitude="-42.81868808" longitude="147.30586807" altitude="19.8M"/> 19.73M
<PolylinePoint latitude="-42.81944949" longitude="147.30498833" altitude="23.4M"/> 20.63M
<PolylinePoint latitude="-42.82021090" longitude="147.30410859" altitude="27.1M"/> 21.51M
<PolylinePoint latitude="-42.82098472" longitude="147.30324241" altitude="29.8M"/> 22.40M
</PolylinePointList>
<PolylineObjectPlacementList>
<PolylineObjectPlacement id="{f5f032a5-6334-4a0b-a854-db0f676e86b3}"/>
</PolylineObjectPlacementList>
</ExtrusionBridge>


The sim is P3D v3.4, using the P3D 1.4 SDK.

Cheers

Braedon

 

[arno]

Hi,

My blog post should help you. You need to calculate the average point height, that's the offset that's added. And then you need to lower it enough so that with the average added you get the right value.

I have all this logic in scenProc, but not yet in the public build.

 

[Bungo]

I thought that was the logic, but with the numbers I have, the vertical difference in length between each of the pylons should be 0.888m, but in practice it requires 3.7m difference between the heights of the pylons in the code to give me that 0.888m increment. If it was a simple offset for the whole bridge it would be easy to compensate for, with the pylons being still 0.888m different from each other. However this example sort of doesn't follow that (ie needing to have one end of the bridge at lower than actual at -3M instead of +14M, but the other end requires an increase in altitude of about 6m (in the other directon) means there's not a simple offset at play here (or am I missing somethng...again... ?)

Thanks

Braedon

 

[Bungo]

I would be interested in trying/testing the extrusion bridge functionality when you get it to a presentable level. It might save me some time. My project is quite large however (the single season PR is a 67GB PSB file, and a 300MB DTM BGL).

cheers

Braedon