As several folks have asked recently about distance measurement
devices I thought a few comments might be in order:

Although I have not tried the devices which you point where you
want to measure, for blind folks it seems likely that such a
thing would be quite difficult to use.  Try pointing at things
whose location you know and asking a sighted helper to tell you
honestly how well you're doing.  I am always shocked how badly I
do at it.

It is inherent in the technology of ultrasonics that
measurements using sound can not have an accuracy better than
about 2 inches in 10 feet.  If this is good enough for your needs
and you can point the thing accurately and repeatedly, then go
for it.

If you are not used to making measurements, or if using a device
that's new to you, have someone check your results with a tape
measure at least the first time or so.  One thing any decent
engineer knows is that just getting a number out of a device, is
no indication the number has any meaning.  If you get the same
number 3 times in a row, and if you know the device to agree with
other devices of known accuracy, then MAYBE! you're O.K.

Measure 3 times, cut once.

There are 3 talking measurement devices available as far as I
know.

1. I believe the device from Maxi Aids is an ultrasonic
measurer, all the above warnings may effect it's usefulness.

2. LS&S group from Chicago sells a so-called talking tape
measure.  This is a little box with a wheel on it that you roll
along to measure a distance.  I have one and find it essentially
useless.  It's inherent accuracy is one quarter inch, they say to
get more accuracy you take several measurements and average them.
I have found it very difficult to get repeatable measurements
within one half inch. .

Also, it does not allow the user to adjust a measurement once
made, there are only 2 buttons, one starts it at zero, the other
reads the current measurement.  You can repeat the reading, but
you can not make a slight correction and read again, you must
repeat the entire measurement.  It would be very difficult to
"find' a desired measurement.The device is mechanically
unstable and very hard to "track."  it's a hundred bucks.

The National federation of the blind's materials center has an
actual talking tape measure.  It is a real steel tape which you
pull from a housing.  It speaks the measurement to one sixteenth
inch when you stop moving it for a half second or so.  It has
memory and other features I have not had time to learn.

Well, the federation has to get it right once in a while <G> It
is also a hundred bucks.  It is obviously the tool of choice for
any serious measurement.

A few of you may know that I spend a lot of research time trying
to develop such a tape.  I was told by machinists that you
couldn't do it the way the engineers the Federation hired managed
it.  Their tape has holes punched in it about every half inch.
Inside the case is a "sprocket" wheel which meshes with these
holes.  This apparently allows a rotating shaft to accurately
follow the movements of the tape.

Machinists told me you couldn't punch holes in a steel tape with
out a serious risk of the hardened tape material breaking.  We
must assume that the engineers who made this tape know better.
Oh well, sometimes even the best sources are wrong.  But not
often I hope.

Hope this adds some clarification

Everything you thought you knew is probably wrong

Tom Fowle
Embedded systems developer/ Rehabilitation Engineer
The Rehabilitation Engineering Research Center
Smith-Kettlewell Eye Research Institute
2318 Fillmore  St.
San Francisco CA 94115
415 345-2123
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