Latitude and Longitude Part II

Another helpful tip when working with latitude/longitude: latitude is measured in degrees, from 0 degrees at the equator, to 90 degrees N (at the North Pole) or S (at the South Pole).

Longitude is measured in degrees east or west of the prime meridian, an imaginary line that starts at the North Pole, runs through Greenwich, England, and goes to the South Pole.

Topo Maps - Contours

Topographic (topo) maps show elevation by contour lines, that is, lines that connect points of equal elevation. A specific topo maps will have contour lines at regular intervals - contour intervals. So the lines might be 10 feet apart, 5 meters apart, or whatever is appropriate for that map.

Azimuth Bearings

Sometimes if you are doing map work you might find yourself expressing a direction using an "azimuth" bearing. That just means you are drawing an imaginary circle around your site, with 0 degrees at straight N, 90o straight East, South at 180 degrees, and West at 270 degrees.

So if you wanted to describe something straight south of your site, you would refer to it as being 270 degrees from your site.

Why wouldn´t you just say "south"? That works well in some cases, but not all. Suppose I am trying to describe a site ten miles from me, on a bearing of 116 degrees - that would be hard to define in words, but using an azimuth bearing, anyone with a map and a protractor could find the spot.

Minutes and Seconds

You can find yourself anywhere on earth if you use latitude and longitude, which are measured in degrees. Each degree is subdivided into minutes and then seconds - so one degree of latitude (or longitude) is the same as sixty minutes of latitude.

That´s why topo maps are often published as 15-minutes quadrangles, or 7.5-minute quadrangles. A 15-minute topo map covers an area 15 minutes of latitude by 15 minutes of longitude.

Compass Bearings

A bearing is the direction on a map from one point to another. Because we traditionally orient maps to north (not east, west, or south), the bearing of a line is usually given with respect to how many degrees off it is from straight north.

So if I want to discuss a property boundary, for example, I might say that the bearing of the eastly boundary of the site is "N 30 degrees E", or "N 10 degrees E."

ManMade Topography

We usually think of topography as hills and valleys formed by nature´s erosional forces. But the highest point of elevation on the Atlantic Seaboard south of Maine isn´t a hill, nor was it formed by tectonics or erosion. It´s the Freshkills Landfill in New York.

Just one more example of man imitating nature. . . .

True North

True geographic north lies at the top of modern maps (although some ancient maps had east at the top!).
True geographic north is the area where lines of longitude converge (since longitude lines run north-south).

But compasses don´t point to geographic north - they point to magnetic north. Depending on where you are on earth, the difference between the two norths can be inconsequential or significant.

That´s why many maps show the magnetic declination at the bottom. It tells you how far magnetic north deviates from geographic north in the area of the world shown by that particular map.

Township/Range Designations

Many parts of the US incorporate the PLS system to divide latitude and longitude into townships, ranges, and sections. A section is a square mile of land, and a block of 36 sections is assigned a township/range number. Townships run north and south of a baseline, and ranges run east and west of a principal meridian. So I can describe one 36-section block of land as being Township 1 North, Range 1 East of the baseline and principal meridian. Within that township/range, the sections are numbered 1 to 36, and can be further divided into quadrants - for example, the northeast quarter of Section 18, Township 2 South, Range 3 West, Seward Meridian.

Latitude and Longitude

Most topographic (topo) maps depict sections of the earth´s surface bounded by lines of latitude and longitude. Latitude and longitude lines are imaginary lines running east-west (those are the latitude lines) or north-south (those are the longitude lines).

That´s confusing, because latitude lines run east-west but tell you how far north or south you are from the equator! Likewise, longitude lines run north-south but tell you how far east or west you are of the prime meridian.

Here´s an old schoolkid trick - remember that on a map or globe, longitude lines are *long* (since they run north-south).

Topographic Maps

Unlike highway maps or political maps, topographic maps show more than locations - they show relief (topography). So with a topo map you can not only find the location of a particular hill, you can see its shape, gauge its steepness, and determine its elevation.