I test terrain following last week,but failed badly,almost lost my plane!There were some problem puzzling me.


First,what's the difference between Absolute and Terrain?I have read "http://plane.ardupilot.com/wiki/common-terrain-following/",it says as bellow 
But it didn't mention what should be done in the MP? Does all I need in the MP is just to choose "Terrain"?

Second, should I both choose the "Terrain" and "Verify Height"? I see that once I choose "Verify Height",the "Alt" will change.but i can't figure out the relationship.

Last, sometimes "no terrain data"will show in the MP if I connect PIX.I think it's beceuse I set "TERRAIN_ENABLE to 1 and TERRAIN_FOLLOW to 1",and no terrain data in SD card. Does that mean that if no prompt shows,terrain follow is ok?

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I don't understand why I need  a "terrain height" for the mission waypoints. And I carefully read the mission file. no matter what I choose as bellow,there was no difference except height value.

I'll answer the easy question :) Absolute is above mean sea level, relative is above home, terrain is according to google maps data sent via MP. However I haven't tested this yet and how instructions will differ when using a laser range finder.

So how about "Verify Height"?Does it same as absolute ?

Absolute is above Sea Level. If you enter 50m it will fly at 50m above MSL until it hits a 75m hill. If launch is above 50m, then it will try to fly under the ground.

Terrain will try to maintain a constant height above the ground underneath it using an uploaded terrain database. This calculation occurs real time inside the Puxhawk, even if not on a flight plan, like RTL.

Verify Height is similar, but the calculation occurs inside Mission Planner only at each waypoint. It simply adds the Google Earth elevation to the requested altitude. If there is a hill between waypoints, it will not know to compensate, whereas TF will adjust. Verify Height is also available to the APM because no onboard data storage is required.

Much of the difficulty and confusion you are experiencing is because the TF feature is not fully implemented in MP. It was developed by Andrew Tridgell who uses MAVProxy instead of Mission Planner.
There are many github requests for terrain following improvements in Mission Planner but they are coming rather slowly. You may notice a new small improvement where the Elevation Profile tool now shows the aircraft following the terrain. There is still a lot of improvement needed to visualize the aircraft's vertical path.
Here are some discussions:


Thanks for your reply.And I have read your post,many problem you mentioned is my question.

Have you ever successfully test TF function?And can you post the TF setting? I badly need this fuction because my area is extremely hilly!


I tested TF using the default settings and it worked very well. Just be sure that the terrain gradient doesn't exceed the climb capability of the aircraft. It works well in hilly areas, but not in mountainous terrain.

I want to fly in mountainous terrain.. What's your setting? Just set "TERRAIN_ENABLE to 1 and TERRAIN_FOLLOW to 1; and select terrain while mission planing,not select verify height and make sure terrain data available in SD card? Another question,my

Yes, you described the settings perfectly. It's really that easy. The hard part is visualizing how the plane will actually fly over the terrain. For example, will it dive into a steep canyon? This behavior is adjusted by the Look Ahead parameter, but how do you know how to set it properly?


another question,MP always prompt me "bad or no terrain data"///But I'M sure I do have the terrain data in SD card//

AND  the prompt seem to disappear later///It's confusing//

Because my flying area is mountainous ///I plan to set "look ahead=500m"///

 visualizing is indeed a question!

What does look ahead para mean.?

For example,if i set look ahead=500 m, fly height=200m, and if the Home alt=100 ,highest point's alt in the 500m distance ahead is 400m. What would happen ?

At the launch point,will it fly at  600m(200m higher than the highest point)?

Or just fly at 400m high  to make sure the plane won't crash into the highest point?

It will try to look at the terrain ahead to smooth out the profile to fly so it doesn't climb and dive at each little bump. If there is a valley 200m ahead and a hill at 500m ahead, it won't start descending if that puts the plane in a position that it cant make the climb to cross the peak at 600m altitude. It uses the TECS climb max parameter to determine this.
So the higher the Look Ahead, the smoother the vertical profile will be. If you are just interested in a safe ground clearance then use a big Look Ahead value. If you want to follow the ground contours very closely, then use a smaller value. There is no way to know for sure if the terrain gradient will exceed the aircraft climb performance. The Mission Planner Elevation Profile is the only way to visualize this.

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