Mapping Hacks

Get Maps on Your Mobile Phone #47

Chapter 5, Mapping with Gadgets

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211

HACK

difference in the 6

th

decimal place is the distance from 0.000000 to 0.

000001 degrees, which is equal to 1/9

th

of a meter. This is equal to about

11 centimeters, or 4.4 inches. Your GPS receiver is not accurate to 4

inches!

Specifying coordinates to four decimal places gives you approximately 10-

meter precision. That is close enough for normal automobile navigation, and

your GPS receiver isn’t really that accurate either. I like to store six digits,

because those last digits obscure any rounding errors that might creep in.

Here is a table of approximate distances by decimal place:

The normal practice is to match your precision and your accuracy, but you

can play games with that convention. The U.S. Census Bureau publishes

their TIGER/Line data with six degrees of precision. When you get out in

the field, the data is often several hundred feet off, implying that at best it is

accurate to about four decimal places. But the TIGER data uses that false

precision to indicate the relative positions of things.

For example, when you give road directions based on your odometer, you

are limited to a precision of 1/10

th

of a mile, which is the limit of your mea-

suring tool. But if you see three houses within the same tenth of a mile, say

2.1 miles from a landmark, you could indicate their relative positions by

adding a false decimal of ...