Calculating the Square Meterage of a Selected Object

[BriniaSona]

I have a grid over my fantasy map.

the grid has 42px by 42px squares.

Each 42px equals 103 meters.

after using the Magic Wand to select my selection i open the histogram and it says the area of selected pixels is 490025.

How do I calculate the area in meters.

Post a stupid mans forum for me. I can photoshop and all, but math is just stupid.

 

[colleague]

((490025 / 42) /42) * 103 * 103 = 2947095 m²

 

[Tom Mann]

a) Each of your "squares" has 42 x 42, or 1764 pixels.

b) You measured 490025 pixels, so divide that by 1764 to get the area in "squares". The result is 277.79 "squares".

c) Each "square" really represents an area of 103m x 103m, or 10,609 square meters.

d) So, you multiply 10,609 square meters per "square" by 277.79 "squares" to get the total area as 2,947,095 square meters.

e) Since 1 square km contains 1,000,000 square meters, this is equivalent to 2.947 square km.

HTH,

Tom M

PS - Just saw Colleague's post. Great minds think alike.

 

[BriniaSona]

crapppp i read it wrong, kilometers not meters, does that change anything?

oh and how do I find out how many kilometers squared a pixel equals?

 

[colleague]

then you can change m² in km²
1 pixel = ((1 / 42) /42) * 103 * 103 = 6,01417233560091 km²
490025 pixels = 490025 * 6,01417233560091 = 2947095 km²

 

[Tom Mann]

@OP: I don't know what area your map covers, but (approximately) three million square km is HUGE. The total surface area of the USA is only 9.85 million square km, so you are talking about an area that is 1/3rd the size of the US.

Are you sure you have the units right?

Tom M

 

[BriniaSona]

the map is large so I'll just link it, Imgur also seems to cut off the right part of the map, but here it is.

http://i.imgur.com/ePzz2YV.jpg

it's a fictional map for my roleplay forum.