How to calculate forest area of a bitmap (jpeg) map?

Question

I have a map of 1:50,000 scale map in .jpg format. Can i calculate the land area of forest,lakes etc from arcmap.Anybody can give me the step to do it? And I want to know what is the coordinate system for sri lankan 1:50000 ,80cmx50cm map?

Answer 1

A digital image of a map where the only information you have is "1:50000 scale" doesn't have a true scale. You need some indication of distance to use a dimensionless scale, and all the image has is pixels. On paper, those pixels are explicitly related to a physical size.

There are some customary standards for relating paper resolution to pixel scale, but they're somewhat specific to the organization making the map, with a chosen number of dots per inch.

The fact that you specify that it is 80cm x 50cm gives you a direct indication of scale, though. 80cm in the longer dimension * 50,000 = 40km in the longer dimension. Divide by the number of pixels in the longer dimension to get the pixel scale, or resolution (eg: 1 meter resolution, a pixel is 1 meter to a side). If forest is represented by green pixels, there are ways to count the number of green pixels, and multiply by the squared pixel scale to get the total area.

What software are you working with?

Answer 2

This is a quick and dirty way to do it: Add some data (such as roads, buildings or water) to ArcMap from the region you know the map is in. Add the .jpg. Add the Georeferencing toolbar and georeference the jpg. Click a control point on the .jpg first and then match it to the same location in the spatial data, like a street intersection or the corner of a lot boundary (if known). You will have to zoom in and out alot. Do this for at least 3 points that are apart from each other. Then use the measure tool to find out an approximate area. The area will not be exact, but close. This is because you've warped the image to fit the data. Or, you can do it the old-school method. Print the map to scale and then use a planimeter to find the area http://en.wikipedia.org/wiki/Planimeter.

Answer 3

Those maps use this coordinate system.

Transverse Mercator projection parameters. 
Kandawala datum. 
Everest 1830 ellipsoid.

Longitude of origin (central meridian) 80° 46' 18.16000" E
Latitude of origin 07° 00' 01.72900" 
Central scale factor 0.9999238418
False northing 200000.000 m
False easting 200000.000 m
Semi-major axis length 6377276.345 m
Reciprocal flattening 300.8017