Repeatable Volumetric Measurements

I recently completed a mapping project for a gravel pit (Steep Rock Pits and Wash Plant). One of the tasks was to measure the volume of the 1" washed gravel pile, stacked by a conveyor belt. I flew the mission and I noted that the subject stockpile would be in the corner of the map. Remembering that edges of the map can sometimes have less "definition" than those items in the center of the map with greater overlap, I elected to fly a second mission over the corner of the pit where the stockpile is located.

I processed both maps and took volume measurements on both. The first map, where the stockpile was on the corner, measured 2700m3 (rounded number for discussion). Ok...

The second map volume measurement of the same pile (keeping the total area of measurement roughly the same) was 2350m3!! What!!??

I know the overlap on the second map, using the subject stockpile as the reference point, was far better than the first one. But would this account for a 10% difference? That is substantial!! Should these measurements, all things considered, not be closer?

Is this a failure in mission planning? Too high? Too fast? More overlap? Should I fly each subject area twice, with the flight paths being 90º to each other? Than render ALL those images together? Or fly the same mission three times, render three models and compare? Every time? I feel that would erode the advantage of using this software.

I believe there is more I can do for mission planning and strategy to obtain the best possible data before I abandon all hope.

Please advise.

Ray Penner March 25, 2016 19:28

Official comment

Good question. It isn't often we get two maps to compare like this.

While all the things you have identified will certainly affect your outputs it is most likely the number and placement of the points used in defining the polygons within which we will measure the volume.

We picked a random stockpile and placed 25-30 points around the base of it and the numbers came out to well within about .5%.

From your Steep Rock WashPlant 20160324 job

SteepRock 1 20160323

This site is actually a really great example of our stockpile measurement tools since the piles are all very well defined and on a level base.

Try more points. We use the points to define the base's shape so the more the better (within reason).

Tudor March 25, 2016 21:56

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7 comments

I have taken multiple volume measurements over the last several days and (hopefully) improved my technique for selection points around the base of the stockpile, typically selecting my points based on the DEM. I am having difficulty achieving results like you show in your example.

I re-reviewed information in your help section in regards to processing and strategies for obtaining the best possible images and I realized that the ground reference image you say to include is the last picture the MME app takes. Prior to this, I took a picture with the DJI Pilot App, than started the MME app. I included "my" ground reference picture. Realizing my error, I reprocessed the Steep Rock Washplant flight with the correct ground reference image and I am having problems getting my volumes to come closer than 8-10% of each other.

Am I picking enough points? Is there something I am missing? Even when the areas are very close together, some of the volumes are way different. In one example, a little stockpile, with similar area, was almost 50% different!

Where have I gone wrong? I want this to work!

Ray Penner March 28, 2016 21:14 0 votes

Your model looks fine and you used good overlap so the issue is not with your data collection. We just made multiple measurements of 4 different piles and were well within 1% on each one.

The top image example above is our approximate point spacing.

Tudor March 28, 2016 21:35 0 votes

It looks like you are a little tight on all of the ones you currently have drawn. This is where using the elevation layer comes in to make sure you aren't drawing the boundary on the dirt in the pile which will thrown off our dynamic base calculation.

Tudor March 28, 2016 21:49 0 votes

Help me understand please... I just measured the same stockpile on my two different Washplant models.

Washplant Model 2 (ground reference image was the last photo taken) has a volume of 129.88m3 with the base area as 138.25m2.

Washplant Model 1 (ground reference image was first photo taken) has a volume of 149.75m3 with the base area as 135.13m2.

That is a 13% difference! This is frustrating as I know you can get your results within 1%, but what am I doing different than you?

Ray Penner March 28, 2016 22:35 0 votes

The smaller the stock pile, the higher the percentage of error it will be since the base errors will come into play more there.

These are the measurements we took from from model 2 and model 1

Yes, not 1% but not 13%...

You don't want to include any points that are selected on the pile so you need to step your polygons off a bit. As long as it is flat it won't matter much.

This is your polygon from Washplant 2. A bit tight and could use more point...

The time at which the ground reference image is taken will only matter if there is significant barometer drift during the flight. Even then, it won't show up in any appreciable way in the model.

Tudor March 28, 2016 22:53 0 votes

Thank you for your help. There is an art that will come with practice and technique involved in choosing the points of the polygon that will measure the stockpile volume.

With persistence and many points clicked, I am able to get my difference between measurements to <5%.

Ray Penner March 29, 2016 22:37 0 votes