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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | Re: GISList: DEM vs. DTM |
| Date: |
04/04/2003 02:10:01 AM |
| From: |
Jayachandran Mani |
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hi ,
DEM and DTM are used sunonymously in earlier days. But
there are conflicting usage of these terminology. I
could define these two in these ways
DEM - Digital Elevation model can be referred to the
continous representation of change in relief of
terrain (height variations of terrains). Here the
third dimension is the elevation (X,Y,Z)
DTM - digital Terrain model can be defined as a
representation of surface other than the elevations.
ie here the 3rd dimension can be other than the
elevation like rainfall intensity, population density
etc.,
that's whay for GIS personnel, if the 3rd dimension is
the elevation, it is DEM and if the 3rd dimension is
other than the elevation it can be referred as DTM.
For the photogrammetrists (who normally works with the
topography and not the attributes associated with it),
the 3rd dimension will always be the elevation. that's
the reason DEM/DTM is used synonymously.
But now, there are few slight differences been brought
into this for the differentiation of the same.
1. When u say a DEM, it is always a regualr gridded
pattern (referred as 10 x 10 m DEM). If the same DEM,
when supplemented with the breaklines (which indicates
the sudden steep changes in the terrain surface) and
spotheights which still give the best approximations
of the terrain surface can be termed as DTM.
This DTM can contain, the bridge, flyover
representaion also.
As far as LIDAR is concerned, LIDAR as a raw data
gives the complete surface DEM or as it is termed by
LIDAR professionals as DTM. for getting the ground DEM
the LIDAR data needs processing and this can be used
for orthorectification. For getting a "true
orthophoto" where ur building relief also is removed
in a better way, ur surface DEM from LIDAR (raw data
can be used. The ortho -photo thus created can still
be improved by having vector buildings / bridges
vectorised in stereo and by using them as additional
features in to ur surface DEM
Any other clarifications of DEM/DTM from members are
welcome.
Regards
Jay
--- Jeff_Boudrie@monroemi.org wrote:
> Good day,
>
> I'm hoping that maybe someone out there may be able
> to help me with these
> questions.
>
> What is the difference between a DEM and a DTM?
>
> Is a DTM preferred for the ortho-rectification of
> aerial imagery and why?
>
> Is LYDAR the preferred method for the development of
> a DTM and are there
> other less expensive ways to do this?
>
> Thanks so much for any help you all may give me.
>
> I will sum.
>
> Thanks,
>
> Jeff Boudrie
> Monroe County Planning Department
>
>
>
>
>
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