A topographic profile is what you would see if you could cut vertically into the Earth along a transect and then viewed the cut from the side. For the moment, focus on the shapes of the profiles i. Drawing a topographic profile is not difficult, but it is important to pay attention to the details, because a topographic profile is often the starting point for more complex analyses.
If the topographic relief of a region is low, a topographic profile drawn using a map scale may be impractical. The map scale may be too small to reveal useful information for a topographic profile. In such a case, the profile can be drawn with vertical exaggeration. Vertical exaggeration is a way of stretching the profile vertically by using a scale that is larger than the horizontal scale from the map.
For example, if you wanted to draw a topographic profile from a map with a scale of , and a 50 m contour interval, then the horizontal lines of your profile would have to be very close together.
Therefore, the horizontal lines would be only 1 mm apart. Plotting the profile would be much easier if the horizontal lines were 1 cm apart. When you use a vertical scale that is different from the horizontal scale, it is common practice to indicate the difference between the scales by calculating the vertical exaggeration VE :.
On your topographic profile you would report that you used a vertical exaggeration of 10 times. A geologic map shows the distribution of rock beds on the surface of the Earth. Geologic maps are often combined with topographic maps. The pattern made by rocks on Earth's surface depends on the orientation of the beds and the topography. Topographic variation often causes the exposure of rock beds to appear in complex patterns, even when the shape of the bed itself is not complicated.
In this lesson you will learn how to draw a geologic cross-section. You will then be able to translate complex surface patterns into a picture of the rocks beneath the Earth's surface. Before we can do that, however, you must first learn how the orientation of planar geological structures is described. In geometrical terms, all structures can be represented as planes or lines.
The orientation of a plane or line in three-dimensional space is referred to as its attitude. In this lab unit, we will focus on the orientation of planar structures.
A planar structure, such as a rock bed or a fault plane, can be described by two measurements. One measurement, the dip , describes the tilt of the plane. Dip is measured down from horizontal Figure 1. Describing the attitude of a planar structure using strike and dip.
The other measurement gives the orientation of a plane in a geographic coordinate system. There are two ways to do this.
One is to specify the dip direction , or the direction in which the plane tilts downhill i. It is more common, however, to specify the strike. The strike of a plane is the geographic orientation of a horizontal line that intersects the plane. Strike is always perpendicular to the dip direction.
There are two systems used to report geographic orientations: the quadrant convention , and the azimuthal convention. Marshak and Mitra illustrate these in Figure p. In the quadrant convention, the geographic direction is given by specifying the angle away from either north or south. The attitude of a plane is most commonly specified by listing the strike direction followed by the dip angle and a general dip direction.
Each of the following are valid ways to specify the strike and dip of the speckled bed:. The long line is oriented along the strike direction.
On a map, you can get the strike from this symbol by measuring the direction of the long line relative to north. The short line indicates the dip direction, and the dip angle is written next to the symbol. These two kinds of maps are often combined. The igneous intrusion represents the special case of a bed with a vertical dip.
You can tell the bed is vertical because both contacts outer surfaces of the bed appear as straight lines, even in the presence of topography. The strike of a vertical bed can be measured directly from the map.
Note that a general dip direction is not required when the dip is vertical. Beds A and B are another special case. If you look carefully at the map, you will see that the contacts of beds C through G intersect the contour lines. This is to be expected if the beds are dipping. However, the contacts of beds A and B do not intersect the contour lines.
Strike is not specified for horizontal beds, because the intersection of a horizontal plane with another horizontal plane contains an infinite number of horizontal lines in an infinite number of directions. The heavy black line along the lower contact of bed B marks an important interval called an angular unconformity. Unconformities are discussed in detail in Unit 2 of the Study Guide. It is called an angular unconformity because it, and beds A and B above it, are not parallel to beds C through G Fig.
Like topographic profiles, geological cross-sections are drawn along a transect. As an example, we will use the transect X-Y as before. Transects are chosen to best highlight the geological features of interest, but most often a transect is chosen such that it runs parallel to the dip direction.
This is because viewing dipping beds in a plane oblique to the dip direction makes them appear to have less of a dip than they actually do. To construct a cross-section that accurately represents the appearance of the beds, would require calculating what the smaller dip angle would be. For now, we will concern ourselves only with cross-sections parallel to the dip direction.
The first step of drawing a geological cross-section is to draw the topographic profile along the transect Fig. Note that the dip angle must be adjusted if the topographic profile is drawn with vertical exaggeration. We will not use vertical exaggeration here. The next step is to mark and draw in the lithologic contacts. The names of adjacent maps or quadrangles are printed at each corner of the map and along each side, to allow you to easily determine other maps in the area that you may need.
Map series, latitude and longitude. The map series type of map is described by how large an area the map covers in latitude and longitude. Fractions of a degree are expressed in minutes ' and seconds " - there are 60 minutes per degree and 60 seconds per minute. Common coverages for U. Geological Survey maps are 7.
Hint: Think area, not distance, and draw a figure of a 7. Scale Scale expresses the relationship between distances on the map and corresponding distances on the ground in the "real world". Topographic maps include both a ratio scale and a graphical scale. Ratio or fractional scales have no units associated, because the same units must be used on both sides of the ratio or fraction.
For example a scale of , indicates that one inch on the map corresponds to 10, inches on the ground or, alternatively, one millimeter on the map corresponds to 10, millimeters or 10 meters on the ground. For the following questions, calculate answers using the numerical scale rather than estimating based on the bar scale.
Remember you are working with squared units here! Measure the map distance from the southern edge to the intersection with I center of two interstate lanes. How long is this stretch of road in miles? Contour lines. A contour line on the map shown in brown connects points of equal elevation above or below a reference plane usually mean sea level, MSL.
These lines allow us with some training to visualize the shape of the land; that is, topography. The contour interval is the vertical difference in elevation between adjacent contour lines e.
Depending on the intended use of the map, vertical exaggeration can be applied to the Y-axis. This can be beneficial in scenarios where the topographic profile is being utilized to show the ruggedness of the terrain.
In scenarios where the primary use of the topographic profile is to project geologic features or cross-sections, vertical exaggeration is best avoided. Topographic profiles can be extremely useful, and provide a starting point for making geologic cross-sections that project rock structures or layers into the subsurface.
In a very general sense, we find that ridges are composed of resistant rocks and valleys are composed of less-resistant, easily eroded rocks. Now that we are familiar with topographic maps and making topographic profiles, let's take a look at how this is carried out.
The first step in making a topographic profile is to obtain a topographic map. These can be generated by the scientist, or gathered from a geological survey agency. Once an appropriate map has been selected, topographic profiling can be started.
Establish a line between two points that intersects the region of interest on the map. These should be labeled as A-A'. Take a strip of paper, and lay it along the cross section line between the two points. On the strip of paper, place a tick mark where each of the contour lines intersects the line of the paper cross-section.
Add notations indicating the elevations of those contour lines. Where there is little topographic variation, mark all contours. If there is substantial topographic variation along the chosen line, begin by only marking the intersection of the major, or "index" contours. These are seen in bold on the map. On graph paper, draw the region of interest. Choose a scale for the y-axis with or without exaggeration.
Set the tick marks along the x-axis and transfer each elevation mark onto the graph with a dot. This generates a graph of elevation versus distance along the A-A' line. Connect these dots to make a continuous line. The map defines the scale of the x-axis, but the Y-axis can be chosen to show a realistic view, or one that accentuates the local topography to demonstrate rugged terrain.
Evaluating the ruggedness or steepness of a terrain can be useful in assessing the difficulty of traversing a particular area. This can be applicable in different ways for different modes of transport such as hiking, biking, or driving. Field work for geological or biological surveys may require making a transect through an area for the purpose of taking measurements or collecting samples. Topographic profiles can inform field scientists of the feasibility and difficulty of sampling in different regions and facilitate planning of an appropriate transect.
Topographic profiles are the land-surface basis for constructing geologic cross-sections. These sections are a graphical projection of surface rock or soil layers into the subsurface, and provide a side view of the Earth's interior crucial to interpreting all kinds of geologic features.
These can be used for many applications, including locating likely sources of ground water reservoirs, possible oil and gas pockets, or regions of folding or faulting. Most topography on Earth is a consequence of the interplay between erosion and uplift, which is caused by volcanism, tectonism, tidal forcing, and impacts. Detailed analyses of topographic variations are a critical part of assessing terrain evolution. You've just watched JoVE's introduction to topographic maps and profiles.
You should now understand the importance of topographic maps, how to make topographic profiles, and how these profiles can be useful to geologists and communities as a whole. Once properly smoothed and checked against the map itself for elevation details between points , the resulting topographic profile is a representation of the highs and lows of a landscape, between the defined points.
When topographic profiles are used as a base for projections of geologic features into the subsurface, it's generally best to avoid vertical exaggeration - in other words, the horizontal and vertical axes should have the same scale. However, when there is very little vertical variation across the topographic profile line, it might be useful in order to visualize topography to have a different vertical scale, effectively stretching out the vertical topographic variations.
The degree of vertical exaggeration is equal to the vertical fractional scale divided by the horizontal fractional scale. For example, if one is using a typical U. Geological Survey topographic map with a horizontal scale of , 1 in. Some vertical exaggeration is often useful, particularly when the topographic profile is being used primarily to show the ruggedness of the terrain. As per the previous example, on a , map the scale generally used on standard USGS 7.
But in many map regions, there is substantially less than 2, ft. For the purpose of making geologic cross-sections, where the main interest is to project rock layers into the subsurface and less significance is given to the surficial topographic variation , it is best to use a non-exaggerated topographic profile as the base from which to make the projections.
With a non-exaggerated profile, the rock layer dip-angles do not need to be modified. This is discussed in greater detail in the "Making a Geologic Cross Section" video. A topographic profile provides a visual representation of the topographic highs and lows across a line segment on a map, from one point to another.
Such profiles are used to evaluate the "ruggedness" of terrain, which is useful in assessing the difficulty of travel driving, biking, or hiking as transportation modes for field-work Figure 1. Sometimes fieldwork requires making a transect through a region for the purpose of collecting samples or making geophysical measurements.
A topographic profile can tell the field-scientist something about the difficulty and feasibility of such a traverse.
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