topic elevation differences between DTMs in 3D Questions

elevation differences between DTMs

StanislavaV — Tue, 31 May 2016 17:47:00 GMT

Hello friends,

i have two DTMs and two DSMs created from LAS files......

first LAS files were created in autumn

second LAS were created in spring

when i compare elevations between DTM autumn and DTM spring.......differences are minimal, but to and fro are big differences, like -50 metres

same at DSMs..

what could have caused it? may it be caused by noise by scanning or even something else?

thank you so much

Re: elevation differences between DTMs

NeilAyres — Tue, 31 May 2016 18:26:33 GMT

Sorry, I am not with you here.

When you say "differences are minimal, but to and fro are big differences, like -50 metres",

is this a difference between the surveys or a difference between the DEM (ground surface) and DSM (includes structures)?

Re: elevation differences between DTMs

StanislavaV — Tue, 31 May 2016 19:11:03 GMT

between the surveys in autumn and spring

Re: elevation differences between DTMs

NeilAyres — Wed, 01 Jun 2016 06:24:05 GMT

Well 50m is a lot. Are these from 2 different survey companies?

There is a possibility that one has supplied ellipsoidal heights and the other geoidal.

Better get back to the supplier to check.

Did they use some local control for their ht measurements?

Re: elevation differences between DTMs

StanislavaV — Wed, 01 Jun 2016 20:26:54 GMT

no they are from one company

that deviations are somewhere in one pixel ....somewhere in two.....can it be caused by noise?

Re: elevation differences between DTMs

SteveLynch — Wed, 01 Jun 2016 20:49:43 GMT

what is the cell size?

what about looking at the raw LAS data?

Re: elevation differences between DTMs

StanislavaV — Wed, 01 Jun 2016 21:55:04 GMT

-cell size is 0,5

-can´t see raw las data ...don´t have access to do that

Re: elevation differences between DTMs

SteveLynch — Wed, 01 Jun 2016 22:21:59 GMT

cell size of 0,5 what, inches or miles or ?

🙂

Re: elevation differences between DTMs

DanPatterson_Retired — Wed, 01 Jun 2016 22:31:45 GMT

or maybe ,50 not -50 Steve

Re: elevation differences between DTMs

StanislavaV — Thu, 02 Jun 2016 05:23:33 GMT

0.5 meters

Re: elevation differences between DTMs

DanPatterson_Retired — Thu, 02 Jun 2016 10:13:08 GMT

So it is 0.5 meters... well that is a whole lot different that 50 meters.

A lot can happen to give rise to a 50 cm = 0.5 m difference in elevation between the fall and winter in an area.

A pile of dirt, a dug hole, error, the list goes on

You will have to elaborate on the area a bit more and perhaps show a map of the elevations and their differences

Re: elevation differences between DTMs

SteveLynch — Thu, 02 Jun 2016 14:44:29 GMT

I understand the problem to be as follows. We have two rasters each with a cell size of 0.5 meters and when subtracting them from each other we see differences of 50 meters.

Re: elevation differences between DTMs

StanislavaV — Thu, 02 Jun 2016 14:57:04 GMT

Yes it agrees. These big deviations are only 1% of whole raster.

Re: elevation differences between DTMs

JodieParkes1 — Tue, 21 Jun 2016 11:34:47 GMT

Hello - I was wondering what process you took to compare two DTMs?

Cheers,

Jodie

Re: elevation differences between DTMs

StanislavaV — Tue, 21 Jun 2016 19:07:39 GMT

hi Jodie,

i used Minus.