Plot Plan Question - Measuring?

I'm assuming that distance measurements as used on plot plans and property deeds are measured horizontally, irrespective of the grade of the land, unless otherwise stated.
I'm not trying to DIY survey my lot, but the question came up when a neighbor showed me how he'd "located" the back of his 100 foot deep lot by running a tape along the ground, said ground having about a 1.5 in 10 slope most of the way.
I told him I thought the measurement should have been made on the horizontal, and by doing it the way he did, he ended up about a foot short of the line he was seeking.
Correct?
Thanks, guys
Jeff
--
Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can smile when things are going wrong, you've thought of someone
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On 23 Aug 2003, Jeff Wisnia wrote:

You are correct! If you consider this problem as a right triangle, he would have been measuring the hypotenuse, which is *always* the longest of the sides. So his measurment should have been greater than 1x the true distance. If you knew exactly how much it dropped over the course of the run, you could calculate the distance he should have measured, using simple trig functions. That's where the surveyors with those long sticks come into play <g>.
--
TP

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