2 messages.--------------------Johnnie
-----------------------------------------
: Fri, 4 Aug 1995 16:44:09 -0500
: [log in to unmask] (Peter H. Dana)
: Re: HORIZONTAL ACCURACY
>----------------------------Original message----------------------------
> Could perhaps John Shanton or some techno-type at USGS
> answer this question? A researcher here wants to know what
> is the best horizontal accuracy one can expect to achieve
> using a USGS 7.5' topo map. He was told by a cartographer
> that the position of, say, a building would come within 6
> feet of a spot on one of these topos. I understand that six
> feet is about fifteen thousandths of an inch on these maps.
> Well, .015 is also the size feeler gauge I use when setting
> the points on my '65 Ford and would be quite a small
> measurement on a topo map. Which leads me to believe that
> there's a limit of accuracy one should expect from a topo
> map. I would suspect that any surveyor, for example,
> neededing an accuracy better than six feet should use a GPS
> instrument. - PML
The following (OCRed so forgive any strange text) is the
United States National Map Accuracy Standard
***********************************************************
With a view to the utmost economy and expedition in producing maps which
fulfill not only the broad needs for standard or principal maps, but also
the reasonable particular needs of individual agencies,
standards of accuracy for published maps are defined as follows:
1. Horizontal accuracy. For maps on publication scales larger than 1:20,000,
not more than 10 percent of the points tested shall be in error by more than
1/30 inch, measured on the publication scale; for maps on publication scales
of 1:20,000 or smaller, 1/50 inch. These limits of accuracy shall apply in
all cases to positions of well-defined points only. Well-defined points are
those that are easily visible or recoverable on the ground, such as the
following: monuments or markers, such as bench marks, property boundary
monuments; intersections of roads, railroads, etc.; corners of large
buildings or structures (or center points of small buildings); etc. In
general what is well defined will also be determined by what is plottable on
the scale of the map within 1/100 inch. Thus while the intersection of two
road or property lines meeting at right angles would come within a sensible
interpretation, identification of the intersection of such lines meeting at
an acute angle would obviously not be practicable within 1/100 inch.
Similarly, features not identifiable upon the ground within close limits are
not to be considered as test points within the limits quoted, even though
their positions may be scaled closely upon the map. In this class would come
timber lines, soil boundaries, etc.
2. Vertical accuracy, as applied to contour maps on all publication scales,
shall be such that not more than 10 percent of the elevations tested shall
be in error more than one-half the contour interval. In checking elevations
taken from the map, the apparent vertical error may be decreased by assuming
a horizontal displacement within the permissible horizontal error for a map
of that scale.
3. The accuracy of any map may be tested by comparing the positions of
points whose locations or elevations are shown upon it with corresponding
positions as determined by surveys of a higher accuracy. Tests shall be made
by the producing agency, which shall also determine which of its maps are to
be tested, and the extent of such testing.
4. Published maps meeting these accuracy requirements shall note this fact
on their legends, as follows: "This map complies with National Map Accuracy
Standards."
5. Published maps whose errors exceed those aforestated shall omit from
their legends all mention of standard accuracy.
6. When a published map is a considerable enlargement of a map drawing
(manuscript) or of a published map, that fact shall be stated in the legend.
For example, "This map is an enlargement of a 1:20,000 scale map drawing, "
or "This map is an enlargement of a 1:24,000-scale published map."
7. To facilitate ready interchange and use of basic information for map
construction among all Federal mapmaking agencies, manuscript maps and
published maps, wherever economically feasible and consistent with the uses
to which the map is to be put, shall conform to latitude and longitude
boundaries, being 15 minutes of latitude and longitude, or 7.5 minutes, or
3-3/4 minutes in size.
Issued June 10, 1941 U.S. BUREAU OF THE BUDGET
Revised April 26, 1943
Revised July 17, 1947
*************************************************************
(Many sources including - here - Thompson. 1981. Maps for America.
Washington: Superindentend of Documents.)
The bottom line is that for a 1:24000 USGS topo sheet the horizontal
accuracy standard is 1/30th of an inch on the map or 800 inches on the
ground. This in meaningful units is 20 meters. And I can't help but add that
it takes at least a Differential GPS code receiver pair (one at a known
reference) to get 20 meters and two GPS survey-grade (carrier phase)
receivers to get much better than 1 meter. Regular old cheap single-receiver
GPS is a 100 meter (2-sigma) system.
Peter
Peter H. Dana - Department of Geography - University of Texas at Austin
Austin, Texas 78712-1098 - Tel: (512) 869-1450 - Fax: (512) 869-0899
-----------------------------------------------------------------------
: Sat, 05 Aug 1995 12:35:25 -0500 (EST)
: William Fawcett <[log in to unmask]>
: HORIZONTAL ACCURACY
> A researcher here wants to know what
> is the best horizontal accuracy one can expect to achieve
> using a USGS 7.5' topo map. He was told by a cartographer
> that the position of, say, a building would come within 6
> feet of a spot on one of these topos. I understand that six
I'm not a wise guy (well maybe I am) but what's a "spot". That, my friend
is the crux of the matter; and if we ignore vertical relationships, what is
the real meaning of a "spot"?
Ok, getting down to business, using a magnifying glass, a 60 scale engineer
rule, averaging multiple runs (throw out flyers) and a ratio tecnique (using
150 second tick marks, I can consistantly resolve a location to within
.1 second ( a "spot" if you please). .1 second is about 10 feet (3 meters) in
Latitude or 8 feet (2.4 meters) in Longitude.
However, that drill ASSUMES the map is perfect. And it isn't!
Be aware that there can be errors in simply making the map: errors in field
data, lens distortion. Also errors in printing. And then errors in using
NAD 27 datum (big errors), which is beyond the scope of this post. And of
course, any projection is a compromise, and that compromise means inaccuracies.
To illustrate this to the layman get a large table and place 9 adjoining
topo quads next to each other. Gee, why don't the corners match up?
Which brings us back to my original question- what is a "spot"? Is your patron
interested in precise location (Lat/Long) or relative placement of objects
within a local area (say the placement of buildings on a college campus)? The
larger the area of interest, the more projection errors come into play. In the
smaller scheme, field data, lens distortion and printing errors become a bigger
problem.
As a pratical matter, I would guess .25 second resolution is the best you could
hope for, which is about 25 feet. I've also heard 20 feet ven as a rule of
thumb.
> I would suspect that any surveyor, for example,
> neededing an accuracy better than six feet should use a GPS
> instrument. - PML
GPS is fine, and a great time saver, but unless it is a professional
unit, corrected to a base station, it won't do any better. Actually, surveyors,
as a rule, need accuracy much better than six feet, and they do it day-in and
day-out with a transit, or some modern incarnation of the same. I suspect that
most contemporary students, enamored with gee-whiz electronics, would be
amazed at the accuracy Geroge Washington achieved with his transit and chain.
And even to this day, precise coordinate location can be made by solar
observation. But, I digress!
Regards,
-Bill
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Bill Fawcett [log in to unmask]
James Madison University fawcetwd@jmuvax (bitnet)
821 S. Main Street, Harrisonburg, VA 22807 (703) 568-3809 voice
N. Lat. 38-26-27 W. Long. 78-52-33 (703) 568-3814 fax
- - Virginia- The home of Matthew Fontaine Maury - -
MFM is not a type of hard drive!
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