-Stage 3.1-
Map Basics
There are 4 main parts to the maps section. The first part is map basics. The next part is topographic maps, the third part is geologic maps, and the fourth is Stratigraphy which is not strictly maps but is associated with geological maps enough to fit in here. All are important to geology and knowledge of each builds on all of them.
What is a map?
A map is a scaled representation of the Earth's surface
Projections
What are the main problems with representing the Earth on a map?
1. The Earth is Round, a map is flat
2. The Earth is 3-D, a map is flat
How do we overcome these problems?
Different types of maps preserve different features of the Earth
No map can preserve all features of a 3-D space on a 2-D map
Properties that can be maintained
Distance
Area
Scale
Direction
Take a look here at the results of different projections on the globe
Scales
What is a scale?
For a map to have any use at all it must have a scale and the scale of the map must be consistent. The scale is how much smaller the map is than the Earth. It is listed in one of 3 ways:
Ratio Scale: 1:24,000
(1" = 24,000" in real life, or 1cm = 24,000cm, or 1' = 24,000' etc)
Fractional Scale: 1/24,000
(Pretty much the same thing as a ratio scale)
Bar Scale: 0 |----------| 1
(A graphical representation of distance with each bar representing a particular type of measurement)
The Legend
The legend on a map is where you find all the useful information on how to read the map, like the scale (above) and what different symbols mean. To the right is the legend from the 2003 North Carolina State Transportation Map.
The main things it lets you know
What the different types of roads are
The distance between 2 points
Whether along a highway
or by using the scale bar
What the other random map symbols mean like "rest area"
Most highway maps will only have a bar scale, not a fractional or ratio scale although it is easy to convert to these:
i.e. 1 inch = 13 miles on this map
1 mile = 5280 feet = 63,360 inches
13 miles = 13*63,360 = 823,680 inches
so the ratio scale is 1:823,680 or fractional scale 1/823,680
no units, they cancel out
Not all bar scales are located in the legend and insert will often have their own scales since they are usually a magnification of the area compared to the larger map
North
Different types of North:
Geographic North - The North Pole. Also directly "up" on a map, maps are always made with north towards the top edge of the map
Magnetic North - The direction your compass points. Since the magnetic north pole is not located at the same location as the geographic north pole (see picture to left). Magnetic north is not a consistent thing so the date that is was mapped is always listed with the magnetic data (WHOI).
Grid North - North for the grid system being used. The type of grid is listed on the map if there is one. Types of grids are described below.
Latitude and Longitude
Latitude and longitude are one of the hardest concepts to grasp in map reading. There are other techniques used, but this is the most common. This is partially because when you become used to using it, you can usually picture the general area in your mind, unlike other techniques where you need to "decipher their code" before it is of any use.
Latitude - Lines parallel to the equator (labeled N-S)
Longitude - Lines that run from pole to pole (labeled E-W, starting with 0 through Greenwich, England)
Labeled - Degrees (o) Minutes (') Seconds (")
Degrees - There are 180 degrees in measuring latitude (90N to 90S)
There are 360 degrees in measuring longitude (180W to 180E)
Minutes - There are 60 minutes in a Degree
When measuring longitude, the closer to the pole you are the closer the distance between degrees, minutes and seconds
Seconds - There are 60 seconds in a Minute
Measuring Latitude and Longitude of a given point (X)
Latitude:
1. Measure the distance between the first two intervals in the N-S direction [A]:
Distance A = 1.0625 inches
2. Count
how many minutes occur in that interval:
Click above for a downloadable copy
Distance A = 48o35'00" - 48o37'30"
Distance A = 2 minutes 30 seconds = 2.5 minutes
So now you have: 1.0625 inches
2.5 minutes
3. Measure the distance to the point you want to measure [B]:
Distance B = 1.327 inches
4. Now you want to determine B in decimal degrees:
So now you have: 1.327 inches
X minutes
5. Now calculate the distance in decimal degrees that occurs over that time by setting the 2 equations equal to each other then cross multiply:
1.0625 inches = 1.327 inches
2.5 minutes X minutes
1.0625 (X) = 3.3175
X = 3.122 minutes
6. Convert to minutes and seconds (if needed):
Multiply everything after the decimal by 60:
3.122 minutes = 3' 7.3"
7. Add the measured distance to the distance that you measure from in B:
(Since we measure down from the larger number we subtract from the top number)
48o37'30" - 3'7.3" = 48o34'22.7"
8. Add label:
(All points in the US are labeled N (because north of equator) and W (because west of England))
Latitude = 48o34'22.7"N
Longitude:
9. Now do everything again in the E-W direction:
C = 122o20' - 120o22'30" = 2.5 minutes
C = 0.8125 inches D = 0.9375 inches and X minutes
0.8125 inches = 0.9375 inches
2.5 minutes X minutes
0.8125 (X) = 2.34375
X = 2.8846 minutes
= 2'53.1"
= 122o20' + 2'53.1"
= 122o22'53.1"W
10. So now the complete latitude and longitude of X is:
(48o34'22.7"N, 122o22'53.1"W)
Grid Systems
Now as opposed to Latitude and Longitude grid systems are used to "simplify" locating places. There a variety of different systems all with their own unique purposes. This is just 3 of the major ones.
Universal Transverse Mercator
This grid system was designed by the military to "metricize" the global location system.
On maps the UTM labels are blue and look like 3724000.
Overall this system is difficult to use and requires 2 charts to decipher what the seemingly random numbers mean
Public Land Survey
More commonly know as "Township and Range"
Most states use this system
The system divides areas into blocks of 36 Townships
These blocks are then broken down into 36 Sections
Labeling is rather simple:
Townships run N-S
Ranges run E-W
Any map with this system on it had a diagram showing how to label the sections
Then sub sections are just based on quadrants (i.e. NW 1/4 S 36)
Labeling Example:
NW1/4, S36, T34N, R9W
State Plane Coordinates
Similar to PLS except it covers the entire 50 states
The system that is used is rather complicated since it is made to reduce errors in measurements
Now onto Topographic Maps
Or skip up to Geological Maps
And related to Geological Maps - Stratigraphy