Monday, May 08, 2006

Google Planimeter and Rain

Huh?? First of all, what's a planimeter? A planimeter is a device which allows surveyors, cartographers and other people with weird interests to quickly compute the area bounded by a number of points by setting the counter on the device to zero, then dragging one arm of the device along the outline of the area while keeing the other arm stationary. Once you come back to the original point, i.e. you complete the circle or loop, you can read the approximate area off the dial. Now, mind you, it's quick and dirty: I wouldn't use it to calculate the amount of concrete in a complicated structural component, as you're liable to have a whole lotta concrete left over. (Quick, where's the nearest pothole?). While working for MTO in the seventies, we used the planimeter extensively to compute monthly estimates on road contracts.

In doing research for converting Latitude and Longitude to UTM, I came across Jeff Poskanzer's web site ( While perusing his site, I noticed one item that piqued my interest: Google planimeter. It allows you to calculate the area for either large or small areas and anything in between, with a good degree of accuracy! How it's done, I don't know, but it sure is interesting. In short, it shows you a Google Map, you place points on it by clicking your mouse and when you think you've got enough accuracy, you read the total area displayed just below the map. Way cool!

So, there was a massive rainstorm in the US on April 29th and 30th (2006). From I copied the map showing the average rainfall expected for the area involved. Then, I used the Google Planimeter involved to compute the total area. As you can see, the area measured 1.292e+6 km2, which equals 1.292 million square kilometers (I only measured the 2 inch rainfall area.) 2 inches equals 5 cm. To calculate the total amount of rain:

1,292,000,000,000 x .05 = 64,600,000,000 cubic meters. That's 64 billion 600 million!

From, I got the actual situation as of about 7:30 am April 29th.


Ronny said...

Hey ome kees,

Ok ... dus als ik het goed begrijp je kunt dit ding vergelijken met een sonar. Zoals op een boot. Je weet aan de hand van de gereflecteerde sonargolven van de bodem naar de boot hoeveel afstand het is. Aan de hand van de lengte maal breedte maal diepte kun je dus de hoeveelheid inhoud berekenen. Niet dat je dat hebt met een sonar want dat is niet interessant maar volgens mij komt zo een planimeter op het zelfde neer.

Je neemt dus een aantal punten en tussen die punten word de diepte gemeten en de afstanden tussen die punten dus lengte en breedte waarmee je dan weer de inhoud en oppervlakte mee kan berekenen geven aan hoeveel inhoud of hoeveel regenval er is geweest tussen die punten.

Am I right???

Keith Hekker said...

You're absolutely right, save for the fact that an oldfashioned planimeter is a mechanical device. So you would have to plot the depths on a piece of paper before you could use the planimeter to calculate the area involved. It is actually used to calculate the volume of water in a pond or a lake, using the soundings (sonar) taken while transversing the surface of the lake with a boat.

Ronny said...

Wicked... best wel slim bedacht .... ik vind dit altijd wel interessante dingen.

Bedank voor de goeie uitleg!!