# The best way to determine changes in volumes

pkroks
Posts:

**2**
Hi,

Apologies if this is a very stupid question. I have been playing around with global mapper 17. I would like to assess two stockpiles on a month to month basis.

What is the best way to assess the change that occurs each month?

i've tried to draw a polygon - digitizer tool - volume analysis cut and fill volumes. then subtract the answers from the new survey from the answers on the old survey. some of the readings were accurate within about 1000 cubes. however there were definitely some inconsistancies.

i used the exact same polygon in both DSM models to assess the volumes and compare.

I have tried to combine the terrain layers and then assess the volume of that surface.

However is it the easier thing to do is just load the first month dsm, load the second month dsm and then draw the polygon around my area of interest - digitizer tool - and calculate volume between surfaces.

each method gives me slightly different answers and i need to be able to replicate and have confidence in my answer.

thanks

Apologies if this is a very stupid question. I have been playing around with global mapper 17. I would like to assess two stockpiles on a month to month basis.

What is the best way to assess the change that occurs each month?

i've tried to draw a polygon - digitizer tool - volume analysis cut and fill volumes. then subtract the answers from the new survey from the answers on the old survey. some of the readings were accurate within about 1000 cubes. however there were definitely some inconsistancies.

i used the exact same polygon in both DSM models to assess the volumes and compare.

I have tried to combine the terrain layers and then assess the volume of that surface.

However is it the easier thing to do is just load the first month dsm, load the second month dsm and then draw the polygon around my area of interest - digitizer tool - and calculate volume between surfaces.

each method gives me slightly different answers and i need to be able to replicate and have confidence in my answer.

thanks

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## Answers

2When I perform a volume analysis - cut and fill on each dsm separately - i then calculate net volume on both dsms and subtract to get the difference. the difference/growth of the pile is 78944 cubes.

when I load the 1st dsm and the new dsm, and perform a calculate volume between surfaces - the cut volume is 74209, the fill volume 8290. so Net volume on that would be 65916 cubes.

if I combine and compare terrain layers - and then assess the new layer of the same area the value i get for net volume is 79000 cubes.

now all these values vary significantly. and i know the actual value is expected to be approximately 72 000 cubes - this is found using historical data along with a simple addition of the volume extracted from the pits on the same mine site.

with the stripping ratio of 9-1 and a total output of about 80 000 cubes from the pits, the waste dump should increase by 72 000cubes.

which value is meant to be the most accurate and why are there such differences?

3I'm having the same issues with stockpile volumes.

I've used the PILE VOLUME function for a particular polygon and then used the same polygon to create a GRID and used "Measure Volume Between Surfaces", which should result in the same answer.

But I am getting 30,988m3 and 26,665m3 respectively - 4,300 cubes different!

Unfortunately, unlike your situation, I do not know the true or even an approximate volume.

Have you had any update on this issue?

Paul