TOPOGRAPHICAL MAP INTERPRETATION
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Definitions of Map
- Map is a diagrammatic representation of an area of land or sea showing physical features, cities, roads, etc.
- Map is a drawing of the earth's surface, or part of that surface, showing the shape and position of different countries, political borders, natural features such as rivers and mountains, and artificial features such as roads and buildings.
- Map is a representation usually on a flat surface of the whole or a part of an area
- Map is a symbolic depiction emphasizing relationships between elements of some space, such as objects, regions, or themes.
|Map of Tanzania|
Map reading is the process of identifying features on a map by using symbols and signs or names. This technical work requires certain skills that any map reader must possess.
Map interpretation is the interpretation of the symbols and signs used on map into ordinary language by indicating the features they represent and draw logical conclusions from the information as represented by the symbols.
Cartography is the study of making or drawing maps
Cartographer is a person who draws or produces maps
TYPES OF MAPS
According to Functions:
1. Topographical Maps. Are maps which show physical features which are natural features e.g. mountains, valleys, hills etc and man-made features e.g. bridges, ponds, roads, settlements etc
2. Statistical Maps. Are maps which show the distribution of things in quantitative manner e.g. distribution of rainfall, temperature, crops etc.. Examples of statistical maps are dot maps, choroploth maps, Isoline maps etc
According to Scale Size:
1. Large scale maps. Are those maps drawn to large scale size e.g. 1:10000 These maps gives a larger representation of small area, they are also more detailed (shows a lot of information). They represent areas like cities, towns and villages.
2. Medium Scale Maps. Are those maps drawn to medium scale size e.g. 1:100000 They show a moderate amount of details. They represent areas like districts, regions and countries.
3. Small scale maps Are those maps drawn to small scale size e.g. 1:1000000 They give a small presentation of a large area; they show little content (little information) They represent areas like continents and the world.
Importance of Map Reading
1. They provide basis for description of geographical phenomenon
2. They are useful for traveling purpose i.e. they guide people to reach their destination
3. Maps are useful for storage of geographical information
4.They are important for field studies
5. They are important for land use planning
6. They are useful for military purposes e.g. during wars.
ESSENTIAL ELEMENTS OF A MAP
The basic skills required in understanding how to read a map include Title, Scale, Key, Indication of the north direction, Margin/Boundary and Date of compilation.
1. Tittle gives the name of the country and the area where mapped. It helps the map reader to know what the map is all about. The heading is usually printed in bold capital used on the map
2.Scale is a ratio between the distance on the map and actual distance on the ground. It is used to find actual distance and areas on the ground. On topographical maps scales are given in form of ratio or lines
3. Key / legend is a feature which explains the signs and symbols which are used on the map. Not all symbols which are used in the key are applicable to the particular map but all signs and symbols applied on the map are shown on the key. Definition of terms which should be added on others terms under the tittle "Basic skills of map reading/Essential elements of a map" are " margins/Bounder
4. Indication of north direction. It gives an idea about the orientation of the map especially in identifying where the north direction is.
5.Margin/Boundary. A frame which borders the map. This guide and limit the map user in reading and interpreting the map. The aim of the margin is to enclose the area covered by the map
6. Date of compilation. Gives the publisher name and when the map was published. This date is important because physical and human settlements features changes with time but the map drawn representing the land does not. For instance between 1960 and 2007 Dar es salaam has changed in many ways.
READING AND INTERPRETING TOPOGRAPHICAL MAPS
Topographical maps are types of maps which describe the physical (natural) and man-made (artificial or cultural) features of a given area. The physical features include relief, vegetation, and drainage, among others. Some of the cultural or artificial features are roads, railways, cities, towns, dams, schools, and many other structures built by man.
Drainage is the plan or layout of the river with its tributaries until it reaches its destination i.e. lake, main river, swamps or an ocean, therefore the concept drainage includes rivers , swamps, lakes, waterfalls ,flood areas
Note: The common drainage shown on the map is rivers, swamps, lakes and ocean. But expect to see even waterfalls especially on coloured topographical map.
This is the plan or layout of the river with its tributaries, or the network displayed by a river and its tributaries. Drainage of the river usually posses different network/system depending on the way how tributaries convey to the main river and the general appearance, hence drainage pattern
Types of Drainage Patterns
The following Patterns are Commonly Displayed in Topographical Maps:
1. Dendritic Pattern
Is a pattern in which its tributaries convey (join) to the main river at an acute angle resembling to the shape of tree trunk and its branches. Dendritic pattern are common in areas of gentle slope and of uniform (homogeneous) rock hardness. Therefore it can be made from granitic or metamorphic rock
2. Trellised Pattern
Is the pattern in which its tributaries convey or join to the main river at almost right angle. This type is commonly found in areas with severe cracks or fractures mostly to the rocks with an alternate hard and soft rock. Therefore this is associated with sedimentary rocks.
3. Radial Pattern
Is the pattern or layout in which its tributaries flow outward from the center (summit) or at the peaks of mountains. Its pattern resemble a spoken ring of bicycles where stream flow out in every direction from the center. Therefore radial drainage pattern is commonly associated with volcanic mountains /region composed of granitic rock or igneous rock
4. Centripetal Pattern
Is the pattern in which almost all streams are following from all direction converging to the center can be to the swamp, lake or depression. The determinant factor of stream flow is a slope .Therefore the drainage pattern can be associated with sedimentary rock.
5. Rectangular Pattern
Is a pattern which resemble trellised, but it has tributaries joining the main river at a right angle. The pattern is common in areas which are faulted. Therefore can be found along sedimentary rocks/granitic rock or any faulted rock.
6. Annular Pattern
Is the pattern with series of streams flowing on flanks or around the dissected dome, depression or crater. Where there are an alternate band of soft and hard rocks. Note: This type is not so common, but is found around Lake Bosumtwi in Ghana .Is commonly found in areas affected by back tilting.
7. Parallel Drainage Pattern
This consists of a series of streams running parallel to one another. In some cases, streams may flow into one big river. It is common in areas of uniformly dipping rocks such as fault scarps or recently emerged coasts. Rivers Athi, Nairobi, Ruiru, Chania, Thika and Sagana in Kenya.
|Parallel Drainage Pattern|
2. ROCK TYPE / STRUCTURE
The surface rock on the topographical maps is not directly indicated. They some clues are needed in order to identify rock type and structure of the mapped area.
The Indicators Used to Show the Type of Rock presented in a certain Area:
1. LAND FORMS
Land forms shown on the topographical maps help on interpretation of rock type for example:
a) The presence of volcanic land form such; crater, caldera, name of the volcanic mountains suggest the presence of igneous rock.
b) The presence of erosion and depositional features such as depression, sand dunes, coral reef suggest the presence of sedimentary rock. Flood plain suggests sedimentary rock.
c) The presence of highland with steep slope indicated that rock are hard and resistance rock that may indicate presence of metamorphic rock.
a) Thick forest suggests the presence of igneous rock.
b) Poor vegetation cover suggests the presence of sedimentary rocks or metamorphic rock.
c) Nature of the rock. Absence of streams indicates that the rocks are permeable. Thus this depict that the rock is soft which can either be sedimentary, limestone or sandstone.
d) Presence of many streams on surface, indicate that the rock are impermeable such as igneous or granitic rock.
Read the contour and the conventional symbols or signs on the map to identify types of relief. Can either be highland relief with lower arts or lowland relief. Describe the relief with associated land forms. When you determine types of relief first look on the units on vertical interval (V.I) whether the unit is in meters or feet. If units are in feet take the highest value the convent into meter finally determine type of relief (1m=3.3ft)
Climate is the average weather condition experience in a given area over a long period of time not less than 30 years. Topographical maps may be used to identify the climate of given mapped area. Some hints used in interpreting climate from topographic map Latitude of the area.
a) Distance from equator used to determine kind of climate of a certain area. For example the area with latitude 5 degrees north or south of equator the area is considered to be in equatorial climatic condition while the area between 5 to 15 N/S is in tropical climatic condition, 15-30 N/S of the equator imply Semidesert 30-40 N/S implies desert climate or Mediterranean climate.
b) Water bodies. a general high density of streams indicates that the area receives high rainfall, presence of low density or seasonal streams, salt lakes and boreholes indicate aridity.
c) Vegetation, presence of forest in the map indicates heavy rainfall while woodland vegetation indicates moderate rainfall. shrubs, thickets and grassland indicate dry condition or light rainfall Crops, crops grow in those areas where water and temperature conditions are favorable for their growth. For example area with tea, coffee and sugar cane indicate that area receives heavy rainfall.
Crops such as cotton, sisal and sorghum indicate medium rainfall Relief/altitude, altitude of features such as high mountain or mountain ranges and plateaus tend to have orographic rainfall and forest making them makes their own mountainous climate. Mountains do also create their own climate due to effect of aspect which creates rain shadow.
5. HUMAN ACTIVITIES
Topographical map may contain information on economic activities undertaking usually the following are shown on topo maps.
1. Agriculture: look on the presence of Rural settlement in absence of other activities scattered cultivation storage houses/center plantation or estate (indicate large scale agriculture) industry such as Ginneres, Hulleries and Decotecator. seldom symbols or signs are used e.g. S-sisal, CC-coffee, Su-sugarcane
2. Pastoralism: look on the presence of cattle market-cattle dips veterinary installation (Vet. Office)/center Creamers (lace where milk, cream butter and cheese are processed Scattered vegetation or dominated by scrub/shrubs) Bore holes (Bore holes (BH) or water hole (wh)
3. Mining: look on the presence of; symbols of PIC () and shovel (salt work quarrying Roasting Sign Tin-TN, Iron-Fe.
4. Fishing: look on the presence of; water bodies such as lakes, seasonal swamps, rivers, dams, ocean. These should be surrounded by settlement.
5. Trade and Transportation; look on the presence of Road, railway, towns and market
|Trade and Transportation|
6. Lumbering; look on the presence of; forest with track-roads ending on their edges. Saw-mill Sao hill forest -Note: thicket, scrubs/shrubs and bamboo trees cannot be exploited as a timber.
7. Tourism: look on the presence of; National parks, Game reserve Recreational centre’s e.g. museum, archives beaches etc Landscape e.g. crater depression etc.
8. Administration: Various administrative activities can be identified from abbreviations on the map. These are given in a list in the margin of the map. They include:
a) Provision of security as evidenced by the presence of a Police Station or Police Post
b) Judicial services as evidenced by the presence of courthouse
c) Other administrative offices such as District Commissioner (DC)
d) Regional Commissioner (RC).
A settlement is a place where people live. It may be as small as single house in a remote area or as large as a mega city.
Types of Settlement
There are two types of settlements which include, urban settlement and rural settlement.
a) Urban Settlement; Is commonly found in areas of District administrative centers, Regional administrative centers, and Capital city of a country.
b) Rural Settlement; Is an area where the majority of people approximately to cover 80% engage in agriculture
It is a layout of dwellings in a particular place. The signs showing settlements on topographical maps are observed to have varied arrangement. The most common pattern include the following:
1. Dispersed Settlement Pattern: It is alternatively called scattered settlement pattern. The houses are widely spaced one to another
3. Linear Settlement Pattern: Houses are concentrated along an elongated objected of economic significance like a road, river, railway lines and others
Factors Encouraging Settlements
1. A reliable source of water supply e.g. presence of permanent rivers, lakes.
2. Gentle slope i.e. people prefer to establish settlement in less hazardous areas
3. Good soil for agriculture
4. Pleasant climate condition
5. Transport and communication.
WAYS OF SHOWING POSITION ON A MAP
A place can be located by its name where it is found. A more accurate way of locating a place is the use of latitudes and longitudes, this method is used by scalars at sea and aircraft in their flights. Generally position of any place can be located by using;
- Place names
- Latitudes and longitudes
- Grid reference
Methods of Showing Relief on Topographical Map
1. Contour Lines
These are lines drawn on topographical maps to join all places or areas with the equal height or attitude from the mean sea level. Contour are very important in determining relief features whether the relief is steep sided or gentle sloping. This measurement of heights shown by contour lines starts from the mean sea level which is regarded as zero height.
This is a point on a map with its exact height fixed usually on a hill top, mountain peak or other visible positions. They are the highest points on any locality. The trigonometrical points are commonly marked by a triangle followed by the numbers indicating the height for example Δ725
3. Spot Height
Spot height is a point on a map with its exact height above a known level e.g. from the
sea level. The position and height of the points have been determined by surveyors. The spot height is marked with a dot followed by the numbers indicating height of the land for example .750
Is also done to show the relief features on the map, different coloring shades on the map to indicate different heights
Huchures are short, broken lines drawn on a relief map showing direction and steepness of a slope
6. Form Lines
Form lines are usually unnumbered lines drawn on a map joining of nearly the same height areas, Are broken lines drawn between two contour lines
Relief Features on Topographical Maps
1. Ridge. Ridge is a narrow and long relief feature with steep slopes on all sides.
An escarpment is an area of highland with very steep slopes on one side and a gentle slope on another side. The steep slope of an escarpment is called the scarp slope and the gentle slope is known as dip slope.
A plateau is an extensive highland region and whose top surface is almost flat. A plateau is easily identified on the map by the absence of contour lines on the higher land surface and with a series of contours close together on either sides.
A slope the inclination or slant of the land. This inclination varies considerably, resulting in the following types of slopes A concave slope are widely spaced at the lower ground and closely spaced at the higher ground. A convex slope on the other hand has a steep slope at the lower ground and a gentle slope at the higher ground. Contours of this slope are closely spaced at the lower ground and widely spaced at the higher ground. However some slopes bear both characteristics, a concave slope is gentle at the lower ground and gets steeper at the higher ground.
5. Col. A Col is the land between two peaks of a mountain or in the mountain ranges.
6. Saddle (pass). A saddle is generally wider than a col. Saddles provide convenient passages across mountain ranges. Contours showing peaks are usually closed.
6. Valley. Valley is the low lying part of the land which is bound over higher ground and steep slope. Valleys are indicated by contours forming ‘V’ shape pointing the higher ground and some valleys have rivers flowing in them.
7. Spur. Spur (Salient) is a projection of the raised land from the side of a hill or mountain into lowland, contours showing a spur form a ‘V’ shape pointing to the lower ground.
8. Hills/peaks. A hill is a rounded upland area not as high as a mountain. Hills rises above the general relative low ground but less than a mountain. Hill height is usually about 350m-650m. Some hills are regular while others are irregular.
A cliff is described as a steep rock face that is vertical or nearly vertical. Cliffs are common in mountainous or hilly areas and along the shores of lakes and seas. On topographical maps, cliffs are shown by contours that are so closely packed that they appear to merge into one another. To emphasize the presence of the cliff, a special symbol is drawn on top of the contours as shown in the figure below.
A plain is a continuous tract of relatively flat land covering a broad area of lowland. Some plains may be raised but the slopes are very gentle. Plains occur as lowlands and at the bottoms of valleys but also on plateaus or uplands at high elevations. On topographical maps, a plain is shown by contours that are very widely spaced. Some rivers, if present, may be seen to have meanders.
A depression on a contour map is shown by contour lines with small marks pointing towards the lowest point of the depression. The first contour line with the depression marks and the contour line outside it have the same elevation.
12. Mountain. This refers to the upland or highland over 1000m from the mean sea level
This used to show variation of relief across a region. The following stages are followed in drawing cross section:
1. Two end points of the area in question are marked AB
2. Join the two points with straight line by a pencil
3. Take a piece of paper measure from point A to B
4. A vertical scale is now required after marking the values of contours on the paper.The horizontal scale of the cross section is in the same scale as that of the map from which the line AB is taken. The highest contour line on the map is 100m
5. The horizontal base line represents sea-level; the marked paper is placed along the base line so that A on the paper falls on A on the scale. Then each contour along the horizontal line is marked with a pencil and ruler, vertical lines are lightly drawn up to the line which represents the contour height.
Vertical Exaggeration (V.E) is a number of times by which the vertical scale is larger than the horizontal scale. In Mathematics V.E expressed, V.E=(Horizontal scale (HS) /(Vertical scale (VS)) Horizontal scale is the map scale/ground scale Both HS and VS should be the same units of measurements when calculating VE For example
if a map scale were 1cm to 100000cm and the vertical 1cm to 100m, the
VE is first converted from 100m to cm VE = 100000cm/10000cm VE=10cm
In reading the map it is important to know from a map whether one place is visible from another or not. In cross section two places A and B were to represent two observation points weather the two places are inter visible or not. To explain inter visibility we look in the cross section if between the points (A to B) a mountain or hill develops we say the two points are not inter visible because the hill is an obstacle When the basin of depression develops we say the inter visibility, as a line of sight when drawn straight i.e. not obstructed But inter visibility can be affected by other factors such as buildings, vegetation etc.
Gradient is the term refers to the measure of slope. Gradient/ slope are measured by comparing vertical distance to the horizontal distance. In map reading the calculation of gradient is done by comparing the vertical interval between two places and the horizontal distance between them.
Vertical Interval (V.I) is the difference in height between the two places; it can be obtained by subtracting the altitude of the lower point from the altitude of the higher point. V.I = highest contour
Lowest Contour The horizontal distance is measured on the map then it is converted into ground distance by the use of the map scale In calculating gradient both vertical and horizontal lengths must be brought to the same unit of length The formula for gradient Gradient =(Vertical interval ) / ( Horizontal distance)
Example: Given; highest contour 700, lowest contour 300
V.I = 400m Length from point A to B is 8.4 cm Scale of map 1cm to 2km Calculate the gradient
Step 1: Change 8.4cm into ground /map scale 1cm to 2km 8.4cm to x =16.8km
Step 2: Change the ground scale into meters 1km =1000m 16.8km =? =16800m
Step 3: Gradient=(Vertical interval (V.I))/(Horizontal distance (H.D)) = 400m/16800m Gradient= 1/42
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