A question about quadrangle maps?

Question

Why would two quadrangle maps with the same scale and same 71/2 minute measurement be different in size?

One map is from Louisiana and the other is Mount Pleasant Michigan. Why would the map in Louisiana have a wider area grid on it than the Michigann one?

Answer 1

Because the degrees of longitude are closer together as you get closer to the poles. So 7.5 degrees in Michigan is going to be narrower than 7.5 degrees in Louisiana. At the north pole the width becomes zero.

Answer 2

A United States Geological Survey(USGS) map is this: the quadrangle refers to the shape of the map it is a polygon with 4 sides = quadrangle. Most USGS maps come in 1:24000 or 1:25000 scale and cover the entirety of the United States. They are cheap usually about 10 dollars. They have a variety of information on them, including contour lines to show elevation. They have a variety of Coordinate systems on them as well, these include the SPS(State Plane System), UTM(Universal Transverse Mercador), and of course Latitude and Longitude. The maps are made by using Aerial Photography with land based surveying.

Answer 3

Because lines of longitude are like slices of an orange. There is very little distance between them near the poles, and much more near the equator.

Louisiana is closer to the equator than Michigan, so 7.5 minutes of arc there covers more ground that it does in Michigan.

Answer 4

They should not be different sizes. If they are the same scale and both 7 1/2 minutes they should be the same size.

You can have different scales on 7 1/2 minute maps. I usually work with 1:24000. Make sure that the printed scale truly is what it is on the map. It could have one scale printed but it might have been reproduced at another scale.

There could be something I am overlooking, but if they are the same scale they should be the same size. All the ones I have worked with have been.

Answer 5

It's called "convergence of meridians."

The distance between 7.5 minutes of latitude is the same everywhere, at least relative to the 1:24,000 map scale of quad maps.

The distance between 7.5 minutes of longitude ~ the distance between 7.5 minutes of latitude at the equator. (not exactly because the earth is an oblate spheroid)

As the latitude increases, the distance between lines of longitude decreases. At the North and South Poles they converge.(the distance between them is zero.)

At a latitude of ~45 degrees, a quad sheet will be about twice as high as it is wide.