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| Subject: | Re: GISList: elevation and contour interval determination |
| Date: |
01/20/2003 09:49:44 PM |
| From: |
Jayachandran Mani |
|
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Hi,
The contours should represent the terrain on your
hardcopy map. The best interval to represent on a 1 :
42000 scale map for a flat terrrain may be around 10 m
interval. for highly undulating it may around 30 to
40m. Or in other words u can say that the two adjacent
contour lines should be able to be clearly represented
on your hardcopy without smudging.
As per the mapping standards, the minimum mappable
unit on the paper is .25mm ie for a scale of 1 :
50,000 map, a dot will occupy 12.5 m in horizontal
spacing. So the minimum planimetric spacing between
two lines should be around 50m in planimetric
position.
But generally the contour interval also gives an
indication about the vertical accuracy. Normally the
one third of contour interval will be the contour
accuracy. So since the contour is created from a DEM
of 1 arc second, your contour accuracy depends on this
factor also.So as explained by Joe, decreasing your
contour interval will not represent the terrain
exactly.
All this depends on the terrain, which cannnot be much
determined only with statistics
Regards
Jay
--- Zachary Fisk <gisuser@hotmail.com> wrote:
> Thanks for catching that error in my description.
> The cell size is one-arc
> second, I've broken down the data of to 1x1 degree
> grids for the US.
>
> My question might be better read as: for a given 1x1
> degree area on the
> earth's surface how can I determine the best contour
> interval at 1:42240
> scale. Best contour interval would show some
> elevation change, not just a
> line running through the map designating 25 feet.
>
> However, for my purposes I should not be putting
> this over area instead just
> look at the standard deviation of the elevation
> change.
>
> ----- Original Message -----
> From: "Joseph Loon" <Joseph.Loon@mail.tamucc.edu>
> To: <gislist@geocomm.com>: <gisuser@hotmail.com>
> Sent: Monday, January 20, 2003 5:28 PM
> Subject: Re: GISList: elevation and contour interval
> determination
>
>
> > Zachary,
> > If you have 1x1 degree data it means that your
> DEM points are about 68
> > miles apart in the US. I very much doubt if the
> accuracy of the data is
> > 25 feet. If you decrease this interval you will be
> manufacturing data
> > which does not exist. For example, if two of your
> data points are on
> > either side of the Grand Canyon they may have
> about the same height - if
> > you then decrease the contour interval you will
> get a fairly flat
> > surface which is, of course not correct. The
> contour interval is a
> > function of the spacing of your data points, which
> spacing should be a
> > function of the terrain type and all this is
> related to the map scale.
> > One of the major objectives of a contour map is
> the portrayal of the
> > terrain (landform), and if the contour interval
> changes over the map
> > this portrayal is difficult to visualize. There
> are ,however, no
> > hard-and-fast rules which connect DEM resolution
> to map scale or contour
> > interval. An example would be the 1 :24,000 maps
> which have 20 ft
> > contour intervals which would relate to DEM
> horizontal spacings of about
> > 60 ft. So I would guess that with your DEM spacing
> of about 60+ MILES
> > your contour interval needs to be several hundreds
> of feet.
> > Joe
> > Texas A&M University-Corpus Christi
> > ============
> >
> > >>> "Zachary Fisk" <gisuser@hotmail.com> 01/20/03
> 10:41AM >>>
> > I've processed DEM data to contour for the US at
> 25 foot intervals by
> > 1x1
> > degree grids. I'd like to change the interval in
> some areas of the
> > country,
> > for instance some 1 mile areas of Texas will only
> contain one 25'
> > contour
> > line.
> >
> > My question is can anyone provide an algorithm for
> determining best
> > contour
> > interval for a given area? I'd like to adjust the
> interval for the two
> > extremes. I've tried taking the elevation
> difference and putting it
> > over the
> > area, and from that get the standard deviation.
> Almost all my data
> > falls
> > within that standard deviation. Am I
> misinterpreting that, or am I off
> > in my
> > methodology all together?
> >
> > thanks,
> > Zachary Fisk-
> >
> >
> >
> > To unsubscribe, write to
> gislist-unsubscribe@geocomm.com
> >
>
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