- If you plan to plot in millimeters, then divide your measurements (poles, rods or perches) by 4.8 (1 millimeter = 4.8 poles). The actual number is 4.772130756, but 4.8 is close enough for most genealogical purposes. The difference is less than the width of a pencil line.
- If you're plotting in inches, then the "divide by" number is 121 (1 inch = 121 poles)
If you need to match your plat to a specific map drawn to a different scale, such as an old county map, or if the distances on the deed are not given in rods, poles or perches, you'll need to calculate your specific scale in order to create a plat.
First, look for a scale in the form of 1:x (1:9,000). The USGS has a handy list of Commonly Used Map Scales along with their relationship in centimeters and inches. You can use this scale to calculate your "divide by" number in either millimeters or inches.
- For millimeters, divide the large number on the map scale (i.e. 9,000) by 5029.2. For our 1:9,000 map example, the millimeter divide by number equals 1.8 (1 millimeter = 1.8 poles).
- For inches, divide the large number on the map scale (i.e. 9,000) by 198. For our 1:9,000 map example, the inches divide by number equals 45.5.
In cases where there is no 1:x scale marked on the map, look for some type of scale designation, such as 1 inch = 1 mile. In most cases, you can use the previously mentioned USGS map scales chart to determine the map scale. Then return to the previous step.