Grid vs Crosshatch overlap

Just a bit curious and thought i’d ask some other people!

I think I’ve always been a bit OTT with my crosshatch overlap. Typically i use at least 75/75, usually, or 80/80 with lots of trees.

Obviously overlap is situationally dependant and there is no one size fits all answer, but considering 75/75 overlap with a grid mission would be fine for terrain data, what would you say the alternative for crosshatch would be? The default of 60/60 seems low, but I know we’re getting nearly twice as much coverage, admittedly from a slightly different angle… so is 60/60 crosshatch a good enough alternative to 75/75 grid?

I usually go out and test these things but time and weather are an issue at the minute!

Thanks

Good morning @Vantage! You are on the right track. I assume by grid you mean what others like to call the lawnmower or snake pattern… I think the most common overlaps for a normal pattern terrain survey would be about 80/70, some like a little higher or lower. I think it is safe to say that if you are doing a nadir crosshatch then you can probably bring that down to 70/60 and if using an oblique crosshatch then 60/60 should be fine. You could actually probably run less than that, but you would have to play with where the threshold is on different scenarios. In most cases it should actually be a little better because of the fact that you are capturing from all sides.

I would rather have more data than less data. If it is crucial that you do it right the first time you cant go wrong with more images. The law of large numbers comes into play at that point. Plus you can always re-process using less images.

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Always true, but you can look at it this way as well. What is the most efficient way to ensure that you do capture enough good data every time? I can run a standard plan like this that I know will work, but what if I want to make it better? Just increasing overlaps is not the answer. Capturing in multiple directions is. This is especially the case when you have obstructions like trees or in this case giant crushing and screening equipment and very steep slopes cut out at the edges. The default plan is 225ft AGL and you can see it has limited the speed to 16mph.

Here is a nadir crosshatch using the older style Auto Flight Mode app. I use this because it allows you to determine the gimbal pitch, which I set at 90. Doesn’t make sense to me why we can’t do that in the new Enhanced 3D, but that is another discussion that I am sure will be had. As you can see the logic automatically reduced the overlaps to compensate so per their recommendation it is doing what I described for you. Yet we still are getting 53 more images… and from different perspectives at the cost of 6 minutes. Also note that the speed has increased.

To take it one step further, since it pushed us just into 3 batteries I raised the AGL 5ft brought the side overlap down 1% and slight adjusted the direction to optimize the changeover between directions and got back down to 2 batteries while still maintaining the original image count. It says 3, but I know from experience it will be two. Sneaky though, think about how many more images I am going to get around the perimeter. Cross-hatch missions also help reduced warping/bowling.