Now let's get to what we actually show on a map. Oftentimes we just use little pictures or symbols to represent different things. On the US Geological Survey map of Long Beach we saw little black squares, rectangles for buildings, blue lines for creeks, stuff like that. Now revisiting the Long Beach map, let's go look on the hill west of the traffic circle. What are these black circles?
Check out the legend. Okay, they are tanks. Actually, they're water tanks. It says so right here. By the way, notice how closely packed together they are.
Even though they're perched on top of a hill... They're not that obvious when you're passing by on the street below. By the way, why does Long Beach pump all its water up to the top of a hill in Signal Hill?
Gravity, from the height of Signal Hill, is what provides pressure to every faucet throughout the city. Cities out in the plains, with no convenient hill available, have to build a water tower to provide that gravity-fed water pressure that's reliably always there, even if power outages shut off electric pumps. Returning to the Long Beach map, let's head over to Wilmington on the left edge of the map.
Here you can see more tanks. From what you know of the neighbourhood, what do you suppose is in these tanks? Oil.
Do you notice that most of these tanks are a lot further apart than the water tanks we saw on Signal Hill? Why do you suppose that is? Well, if one tank were to catch on fire, we wouldn't want the fire to spread to nearby tanks. Take a close look in between the tanks. What do you see?
Rows of little short brown... cross lines. What do you think they represent?
Well brown is used for topography elevation. These are berms, ridges of dirt between the tanks. Right here the ridges have been covered by black asphalt. What are they there for? So that if a tank leaks the escaping oil is securely pondered in a small area, it doesn't flow away to contaminate the environment and maybe get into nearby waterways.
What I want you to get from all this is the incredible wealth of detailed information that these maps provide you with. Often we use shaded in coloured areas to represent different things. Take a look at this map entitled World Map of Natural Vegetation.
It's in your textbook located somewhere in the middle of chapter 11 around page 300 depending on your edition as figure 1123. The land areas of the world are all coloured. The different colours represent different types of natural vegetation. I know the legend is kind of small, but I want to draw your attention to the pink color, which represents desert, and the yellow color, which represents tropical grassland.
Now let's go look at Africa. Here is a large area of pink desert and lying south of it an area of yellow. tropical grassland.
You can see a nice black line separating the pink area from the yellow area. If you went to that part of Africa, is there a place on that line where you could stand and look one way into the desert and look the other way into the tropical grassland? Absolutely not.
In reality, there's probably like a hundred mile wide zone there. On the north side of the zone there's pretty much no vegetation. On the south side of the zone you'd say you were in grassland. In between the vegetation just gradually gets sparser and sparser.
So in reality there shouldn't be a solid black line there. There should be a fuzzy area that grades from pink to yellow. Thing is, the map would be much harder to read and much less attractive if all the boundaries were broad fuzzy bands.
But what you need to realize is that the black lines on maps like this are essentially drafting conveniences and don't mean anything in the real world. Now I'd like you to look at this map, which you'll find in your textbook on what is called the end paper, the inside-back cover. It also uses colours, but it's rather different.
This one is entitled World Map of Population Density, and the different colours represent numbers of people per square mile of real estate. I'd like to draw your attention in particular to the orange color representing 25 to 50 people per square mile and next to it the bright yellow color representing 3 to 25 people. Notice also the light sandy color above that which is under three people per square mile. Now let's focus on those colors in the central part of the United States. Here's that orange colour representing 25 to 50 people per square mile.
Here's the bright yellow representing 3 to 25 people and over here further west is the light sandy colour representing under 3 people per square mile. These are some pretty big ranges, 3 to 25 people, 25 to 50 people per But notice if this yellow area is less than 25 and this orange area is more than 25, how many people per square mile are there on the black line here? 25, no more and no less.
Less on this side, more on that side, 25 on the line. And by the same token, if there are less than three people on this side of this line, and more than three people on that side of the line. On the line itself there are three, no more and no less.
So the lines are actually giving you more accurate information than these big coloured ranges. If we had more data available in here we could probably draw in a 20 line and a 15 line and a 10 line. Okay, so on this map here, the lines are the good data, and the colors are just there to make it easier to appreciate what the lines are telling you. So these are very special lines.
They are members of a class of lines that we call ISO lines. ISO is a Greek prefix meaning equal, so these are lines of equal something. This is a line here of equal numbers of people per square mile. Common ISO lines that we use are ISO therms.
What do you think those are? Therm, thermometer, temperature, lines of equal temperature on the weatherman. ISO bars, bar, barometer, air pressure, again on a weather map. ISO baths, depth of water in a lake or the ocean.
But probably one of the most important, one of the most commonly used ISO lines on a map, we've been using them for a long time, and we don't even use the ISO term, we call them Contours. A contour is a line of equal elevation above or occasionally below sea level. And a map that shows contour lines is called a topographic map. Okay.
US Geological Survey maps, like the Long Beach Quadrangle map, are topographic maps. The contours are the brown lines running across the map. How do contours work? Well, visualize this cute little volcanic island surrounded by the ocean.
All along the shore, where the ocean meets the island, is elevation zero. By definition, sea level is elevation zero. Let's draw in that coastline on the map at the top and label it zero.
Now let's imagine that global warming gets totally out of control and sea level rises 50 feet. Does the island get larger, get smaller, or stay the same size? It gets smaller.
Here's the new coastline. Let's mark that on the map and label it 50. Now we raise sea level another 50 feet to a total of 100. The island is still there but it's very small. We mark the new coastline on the map and label it 100. So now There, up at the top, is our complete contour map.
0, 50, 100 feet. Now I'm going to remove the island, leave the map, and ask you some questions. First, where is the summit, the highest point on the island, located? Well, it's inside the 100 ring, about here, more or less in the middle.
Next question. What is the elevation at the summit, the point where the flag is located? Well, it's on the side of the 100 line, away from the 50 line, so that means it's more than 100 feet.
How much more? Could it be 200 feet? Well, a critical rule for contours is they always come in even increments.
We have here 0, 50, 100. What's the next one in the series? The next one is 150. We don't have a 150. Therefore, the highest point is less than 150. Could be as much as 149, but it can't be 150 because there isn't a 150 contour. The contour that isn't there is just as important as the ones that are.
How do we use contours? We can use them for determining elevation. What is the elevation at point x on this map? Well, it's not 60 and it's not 80, it's about halfway in between. Let's call it 70. Do we know that for sure?
No we don't. We don't have details about what's going on in here. What assumption are we making when we say it's 70?
We're making the assumption that the ground slopes evenly from 60 to 80 here. But you know, there's the 60, this is a profile, there's the 80. We're assuming that the ground goes like that. The ground might go like that with a big cliff, where point X is here.
So it's like 79. So it can't be 80. Or the ground might go like this. And point x will be down here, 63, 65. But you don't know that. That information is provided, so yes. The answer to that question, what is the height at point x, would be between 60 and 80, greater than 60 and less than 80. But any number in that range is in fact an okay answer.
70 would be what we would usually pick, yes.