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The effect of constand AGL capture

Zane, I am having a conversation with someone about best practice while collecting imagery for SFM processing.  He put forth that to aid in overcoming problems associated with non-calibrated camera, parallel image capture (doming effect), he sometimes gathers images at 2 different altitudes, like a 50' difference, and includes them in the process. My initial reaction is that that is a bad idea, but I don't have a lot of knowledge about the trig. involved in processing.  What is your take?

Dave Pitman

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It all depends on your starting point and how well you are going to be collecting the data. We offer the terrain awareness feature because doing flat flight at low altitudes (less that 400 ft to be legal) in undulating terrain will either cause there to be not enough overlap in the high areas or way too much in the low areas. This causes a large GSD mismatch and the excessive overlap causes processing inefficiency. 

People frequently underestimate how much terrain and structures will affect the achieved overlap. They they have bigger problems than doming. We optimize for the map layer being correct first before the elevation layer so the user gets something useful. 

There are all sorts of tricks people try to play to get rid of doming but we have found that the best way to do it is to use lots of overlap. We have a readjustment stage in our processing that is expressly intended to correct for any detected doming. 

Flying a second low overlap pass over an area is helpful but will harm the output GSD while only adding minimal benefit. Those images would be better spent adding to the overlap of the target flight altitude. 

Zane
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Okay so processing images at different AGLs is more detrimental to GSD than it is to geometric accuracy, assuming overlap is good throughout the whole set. And Terrain awareness is aimed at sustaining good overlap and improving GSD than improving Geo accuracy. Would that be correct?

Thank you, Zane

Dave Pitman 0 votes
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Those statements are accurate. Terrain Awareness is not there to increase any sort of accuracy. It is there just to make the map work at all by maintaining consistent overlap. 

Zane 0 votes
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