X-Mailer: ELM [version 2.3 PL11]
Newsgroups: comp.infosystems.gis
Date:         Wed, 14 Oct 1992 17:47:01 CDT
              <GIS-L@UBVM.BITNET>
From:         "(Tim McGrath)" <tmcgrath@TMCGDNELNC.CR.USGS.GOV>
Subject:      Re: Confused about digital scales
              at Oct 14, 92 1:19 am

> From: "(Ian S Worthington)" <isw@VNET.IBM.COM>
> Subject:      Re: Confused about digital scales
>
> >
> >The scale 1:50,000 *does* have a physical reality, and that is the reality of
> >the map that was originally digitized, which should be traceable back to a
> >representation of actual ground features.  Using the scale as a measure of
> >accuracy carries certain assumptions:
> > .
> > .
> If I had a database which had a stated accuracy of, say, 25m, I would
> know that, if I reproduced a map on a monitor, then I should not allow
> the user to zoom in to a greater level of detail than 1 pixel = 25m.
> Similarly, if I plotted, or printed, from this database, the maximum
> size I should allow would be <the minumum reproducable physical
> distance> = 25m.  I think this is correct, yes?
>
> So, if instead of this I use a database with a given scale of 1:50
> 000, how do I determine how 'close' to let a user zoom in?  Until the
> 'virtual' screen/page is 50 000 pixels/points, maximum?
>

We need to discuss all three of the factors involved with digital map
accuracy; 1) Scale, 2) accuracy (relative to ground coordinates), and 3)
resolution.

A given scale IS a "zoom" factor.  The units you use are entirely up to you.
A scale of 1:50,000 would be 1 inch on the screen to 50,000 inches of the
data being displayed.  If you need to use pixels you'll have to find out
the pixel size of your monitor in units relative to your data.  Letting the
user "zoom in" means changing the scale -- no two ways about it.  A "small"
scale of 1:2,000,000 shows "small" detail, a "large" scale of 1:24,000
shows "large" detail.

Scale can be used as a measure of accuracy only in cases where a published
scale has a corresponding published (i.e. measured) accuracy.  In 1941 the
U.S. Bureau of the Budget (a well known mapping agency eh?) issued the
"United States National Map Accuracy Standards" for all Federal agencies
that produce maps -- the current version was issued in 1947.  The standards
are available along with more information in a flyer titled "Map Accuracy"
from you friendly neighborhood ESIC (Earth Science Information Center)
at 1-800-USA-MAPS.  According to the brochure, "the horizontal accuracy
standard requires that the positions of 90 percent of all points tested must
be accurate within 1/50th of an inch (0.05 centimeters) on the map.  At
1:24,000 scale, 1/50th of an inch is 40 feet (12.2 meters).  The vertical
accuracy standard requires that the elevation of 90 percent of all points
tested must be correct within half of the contour interval.  On a map with
a contour interval of 10 feet, the map must correctly show 90 percent of all
points tested within 5 feet (1.5 meters) of the actual elevation."

So for 1:24,000 DLGs the coordinates should be accurate to within 40 feet
(12.2 meters) on the ground.  Now we need to discuss resolution.  USGS DLGs
have a resolution corresponding to the smallest data collection unit
(0.001 inch) for each scale.  That is, 0.001 is the accuracy of the of
digitizer or scanner.  Quality control is done by comparing with proof plots
of the original source material generated with an automated drafting
machine with a resolution of 0.001 inch.  There is no quantification of
operator error.  The following table is from page 19 of the "Digital Line
Graphs from 1:24000-Scale Maps, Data Users Guide 1":

Source Scale     Resolution
------------     ----------
1:24,000         0.61 meter
1:25,000         0.635 meter
1:48,000         1.22 meters
1:62,500         1.587 meters
1:63,360         1.61 meters

For a 1:24,000 DLG there is an accuracy of 40 feet (12.2 meters) and a
resolution of 0.61 meter. The USGS National Mapping Division has rigorous
QA/QC standards for digitizing maps. If you're trying to estimate accuracy
of in-house digital data you need to take into account operator error in
either manual digitizing or registering of scanned data.  You also need to
include information about the base material of the source.  Paper maps are
not a scale stable base material.

Many GIS people imply a map accuracy when they just use a scale -- but if
the source material does not conform to a published or tested standard
than it can be difficult to "guess" at the accuracy.

> If so, isn't it of more use to quote the minumum *accuracy* of the
> data to start with?
>
> Ian S Worthington
> ALCS Development
>

Hopefully it is most useful to include ALL metadata for your spatial
information, including accuracy, resolution, and source scale.


--
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Tim McGrath                   | "Without Geography, where would you be?"
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U.S. Geological Survey        |
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