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| Introduction | Mapping | Local Geology | |
Regional Geology |
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Positioning (Cont'd) |  |
The Earth's shape is that of a ball or globe. To go around the equator and return to the same point, one would pass through 360 degrees of angular separation and each degree of longitude would be crossed at equal intervals. To circle the globe along longitude start at the equator on the vertical straight line; this is called the Prime Meridian, in the middle of the image. Then go south to the pole; then follow the outside line on the right side to the North Pole. Then go south along the same middle line back to the equator. You just circled the globe! Now how can you circle the globe along latitude? At the equator start all the way to the right and then go all the way to the left side. Now you can see how flat maps can represent curved surfaces.
Latitude and longitude degrees are further divided into minutes and seconds.
Each degree has 60 minutes (60'), and each minute has 60 seconds
(60"). Virginia City and the Comstock Lode are roughly positioned at
a longitude of 119 degrees, 39 minutes west (119° 39' W) of the Prime
Meridian and at a latitude of 39 degrees, 18 minutes north (39°
18' N) of the equator. When using seconds, specific points or features
can be accurately located, such as the courthouse or a certain cultural feature
like a mineshaft.
The upper left corner of the Virginia City topographic map is shown
above. Latitude and longitude mark the exact coordinates of each map corner.
To the right on the image, 42' 30" is posted above a tick mark, this
mark locates the position of 119 degrees, 42 minutes, 30 seconds west longitude.
The same longitude tick marks show up on the bottom of the map. All
around the map edges there is either a latitude or longitude tick posted
every 2.5'; these, coupled with the map edges, define the map's latitude
and longitude grid. Longitude is always posted at the tops and bottoms
of these maps, while latitude is posted on the sides. (The north
arrow points to the top of the
map.)
Note that
the 39° 22' 30" latitude line is shown in the corner of the map.
The upper right corner of the Virginia City Topographic Quadrangle map is show to the right. The latitude and longitude coordinates are posted for the map corner. What number changed between the two upper corners, latitude or longitude? For the numbers that changed, their difference is 7.5'. Between the upper and lower edges of the map a similar latitude difference of 7.5' occurs (also see the images below). Because of this the map is titled, "7.5 Minute Series (Topographic)". In other words, the USGS has produced a series of maps that have boundary clips every 7.5' . They also produce other map series with 15', 1° and 2° clips; these maps cover progressively bigger areas, but become less detailed as they get bigger.
Shown here are
the bottom corners of the map. Can you predict how many latitude plus
longitude coordinate grid lines are on the map, 2,
4, 8, or
12? Can you determine how many single
coordinate tick marks are on the map, 6,
9, 12, or
16? Finally, if you connected all of the opposing
tick marks with lines across the whole map how many lines, including the
map edges, would you have for establishing a grid 6,
8, 10, or
12?
Activity #1. Using the topographic map that you made on notebook paper in the previous lesson, create a latitude and longitude grid system for your map Do this for a position south of the equator and east of the Prime Meridian (the opposite side of the world). Model your grid to the 7.5-minute topographic map example shown above.
Activity # 2. Visit the site listed below and make a 15-degree series map based on latitude and longitude coordinates that includes Viriginia City, Nevada or the place where you live.
http://www.aquarius.geomar.de/omc/
End of Lesson
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Positioning (Cont'd) | |
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