Topographic maps

Topographic map example with contour lines.

Topographic map section of Nablus area (West Bank) with contour lines every 100 meters. Colors indicate height.

The topographic maps are an essential element to get into nature. Having a good mountain map will allow us to get to know new places with safety (es), as well as to reduce the risks in places that are already known to us.

This manual deals with topographic maps in general: general concepts, usage and presentation of various maps on the market. You will learn how to interpret topographic maps and also how maps are produced today. If you want to purchase, consult or download mountain maps (es) and map viewing software you can go directly to the Mountain maps (es) page. Please note that only some basics will be covered, this is not a topography manual.

Introduction

A topographic map is a type of map that is characterized by presenting the relief with a level of detail at a large scale, using for this purpose the so-called contour lines.

The Canadian Centre for Topographic Information defines a topographic map as a detailed and accurate representation of the cultural and natural features of the terrain.

Some authors differentiate topographic maps from other maps, such as chorographic maps that are smaller in scale and cover larger areas, planimetric maps that do not show elevations, and thematic maps that focus on a specific feature.

In addition to contour lines, these maps present other information about the terrain, such as vegetation.

Topographic maps have multiple uses today: any type of large-scale geographic planning or architecture; Earth Sciences and many other geographic disciplines; mining and other terrain-based work; and of course, recreational uses such as hiking and mountain orienteering, employing highly detailed maps for this purpose.

Features

Contour lines

Topographic maps show the elevation of the terrain by means of contour lines. Contour lines are curves connecting contiguous points that are at the same height above sea level. Thus, all points marked on the 100 m curve are at an elevation of 100 m above sea level. In Spain the level is measured over the Alicante sea.

Conventional symbols

The many features shown on the map are represented by signs or conventional symbols. For example, different types of roads can be indicated by the use of different colors. These signs usually appear in the legend of the map.

Typically these maps not only show elevations, but also include any rivers and other bodies of water, forests, built-up areas or individual buildings (depending on the scale), and other features and points of interest.

The background color of a topographic map, for example, can be used either to indicate elevation (with dark brown colors for higher elevations and green for lower elevations) or to indicate the type of terrain (forests, for example). The first use simplifies the understanding of the map but is redundant, since the elevation information can already be deduced from the study of contour lines.

Scale

Maps, which are representations of reality, cannot show the elements in their actual size, but a scale is applied, so that large surfaces can be represented on a manageable map that respects proportions, much smaller in size than reality.

Every map must indicate the scale at which it is made. A very common scale is 1:50,000, which indicates that one unit of the map is 50,000 units of reality. Thus, 2 cm of a map at a scale of 1:50,000 is 1 km of reality (2 cm x 50,000 = 100,000 cm = 1 km). In addition to being applied to distances it can be applied to surfaces (2 cm² of the map are representing 2 km²).

To calculate the real distance, i.e. the conversion between map distance and real distance is done by multiplying, and dividing we convert the real distance into map distance. Keep in mind that we will always obtain results in the units in which we have taken the measurements.

The larger the denominator, the smaller the final map will be, since each centimeter of the map allows to cover a larger area.

We speak of small scale when we obtain a small map (for example, 1:1.000.000), and large scale when we obtain large maps (for example, 1:25.000). A very small scale allows to represent the whole world in a sheet.

The scale used in a map depends on the function of the map, since different scales allow us to study different phenomena.

• At scales of 1:1,000 and 1:5,000, phenomena of great detail can be studied, since houses, for example, can be drawn. They receive the specific name of planes, since at such a large scale a projection is not necessary and the Earth can be considered flat.
• With scales between 1:5,000 and 1:20,000 we can represent street plans of cities.
• Between 1:20,000 and 1:50,000 we can study counties and municipalities.
• Between 1:50.000 and 1:200.000 we can study provinces and regions, and roads.
• Between 1:200,000 and 1:1,000,000 we can see autonomous communities and countries.
• At scales lower than 1:1,000,000 we can see continents and even the whole world.

The most commonly used map in geographic analysis is the 1:50,000 scale. Smaller maps allow an overall view, and larger ones allow greater detail. This is the size used, for example, in the National Topographic Map of Spain.

Representation of the altitude

The difference in altitude between two consecutive contour lines is called equidistance. In each map the equidistance can vary, but in those of scale 1:50,000 it is usually 20 meters. Every four or five curves (that is to say, in our case, every 100 or 120 meters) the contour line is drawn thicker and receives the name of master curve. The altitude above sea level is indicated on these master curves, so that for the rest of contour lines it can be deduced knowing the equidistance.

It is common to find maps on which the altitude of a specific point is indicated. The point that is accompanied by the value of the altitude above sea level is called elevation, and is used to report the altitude of particular places, such as lakes, since only with contour lines it would only be possible to estimate their altitude.

The contour lines allow to know if a route is ascending or descending, looking at a contour line, and checking for which side the altitude increases and for which side the altitude decreases. Learning to correctly interpret a topographic map is a matter of practice, and once started, the mountaineer will be able to understand at a glance at the map the layout of the orography: mountains, valleys, slopes, rivers...

A path cuts through multiple contour lines. Keep in mind that each time it crosses one of these curves, the path has gained or lost the number of meters indicated by the equidistance. The closer together the contour lines a road crosses, the steeper the slope will be. On the other hand, a road parallel to a contour line will have a zero slope.

There are some typical figures that correspond to common landscape elements, the significant ones are: the valley, the ridgeline or dividing line and the pass.

Representation of a valley

Representation of a valley

Representation of a ridge

Representation of a ridge

Representation of a mountain pass

Representation of a mountain pass

The calculation of the slope

The slope is the ratio between the slope we have to overcome and the horizontal distance we have to cover. The horizontal distance is measured on the map, while the vertical distance is the difference in altitudes, which can be obtained from contour line information (see altitude representation). The slope is expressed in percent or in degrees.

Slope in percent

To calculate a slope in percent, simply solve the following rule of three:

 Distance in horizontal --- 100
 Distance in vertical --- X

Then:

 Slope % = ( Vertical distance - 100 ) / Horizontal distance

This implies that a 100% slope is a climb that gains the same height that we have traveled horizontally.

Slope in degrees

To calculate the slope in degrees just solve the right triangle with the two known legs (horizontal distance and vertical distance).

Slope º = tangent arc ( Height/Distance )

In this case, a slope of 45º is a slope of 100%.

Planimetric distance and real distance

When measuring a distance on the map we are not measuring the real distance but a horizontal projection called planimetric distance. It is easy to understand that an uphill or downhill road is represented the same as a flat road, and that the distances do not coincide except in the case of the flat road. The difference between the planimetric distance and the real one can be considerable if the slope is steep.

To calculate the real distance, the value of the hypotenuse of a right triangle is found. The leg represented horizontally is the planimetric distance, the leg represented vertically is the difference in altitude between the two points, and by finding the value of the hypotenuse we will find the real distance. The three distances must be in the same unit (meters or kilometers).

The formula to clear the hypotenuse is deduced from the Pythagorean theorem:

real distance = square root ( planimetric distance2 + height difference2 ).

The planimetric distance is measured directly on the map. If the route is not a straight line it can be measured either with a string, or with a succession of small straight lines, or with an instrument called a curvimeter. The difference in altitudes is deduced from the information provided by the contour lines if we do not know the exact altitudes at each point.

It is important to note that this way of measuring the real distance is a simplified model that is not accurate in practice, because although we can accurately measure both the planimetric distance and the difference in altitudes, the various slopes on a road are not taken into account, but their average is taken, so that part of the real effective distance is lost (see below the (topographic cuts that solve this limitation).

The topographic section

The topographic cut is used to understand how the relief is in a very particular part of the map. If we imagine the landscape as a model, we can say that a plan is a photograph taken from above, while the topographic cut consists of a cut of the model (a physical cut, as if we cut the model in half) and to draw the part of the model that has been cut.

To make a topographic cut we must start from the information provided by the map, i.e. the contour lines, the horizontal distance between two points and the scale.

The process to elaborate a topographic cut is the following one:

1. Two points are selected on the map and a straight line is drawn between them.
2. On a paper placed on top of the line we mark all the contour lines that we find. If the curves are very close together, only the contour lines can be taken.
3. On the paper, we draw a coordinate axis.
   • The horizontal axis (abscissa axis) will have the same scale as the map, although we can change it by applying the corresponding calculations. On that line we place the distances between the contour lines we have on the other sheet.
   • The vertical axis (ordinate axis) will have a different scale. You can choose the scale that best reflects the changes in altitude, so that by placing the lowest point near the bottom of the axis, the highest point is also near the top of the axis. It is usual to use the 1:10,000 scale because it allows to see changes in the relief, so that 1 cm of paper is 100 meters of reality.
4. Each point marked on the horizonal axis is raised vertically until it coincides with the corresponding altitude and marked.
5. Finally, we join all the points and obtain the relief profile in a straight line between the two selected points.
6. In addition, the cut can be completed with information such as the sheet on which the selected area is located, the name of the points at the ends of the cut, and if possible the name of the elevations, the rivers and villages through which it passes, the scale we have used and the direction of the cut.

The cut is not always made in a straight line, since it is possible to make the cut of a winding path (such as the route we will take or the course of a river). The same technique is used for this, only that the distances from one contour line to another are not taken in a straight line, but the desired path is followed. The topographic cut of a route allows us to know the slopes that we will suffer while walking, and this technique allows to detect with comfort if a route is flat, if it has slopes, in which moments the worst ascents will arrive...

A second alternative is to make several cuts (in straight line) parallel and highlighting the lines that stand out we will have a composite cut, which gives us an idea of the aspect of the landscape.

Computer programs that visualize digital maps allow to generate topographic slices in a fast way, simply by indicating to the application the points that form the path.

The direction and orientation of the map

When using a map, it is important to know both the place on the map where we are and its orientation, that is, to know where we will move on the map if in reality we would start walking in a certain direction.

To place ourselves on a map we must be in a known place, at the intersection of two lines on the map that we know what they correspond to in reality. In a street map it is enough to place ourselves at the intersection between two streets to locate on the map the point where we are. In the mountains this can be more difficult, but we can use different techniques:

• We will usually know where we are, at least approximately, by the situation. If we have climbed Gorbea, for example, locating us on the map is as simple as looking for that peak. Or if we are walking along the GR 11 and we have left Candanchú towards ibón de Estanés, by the time we have spent we can know approximately the area where we are.
• By identification. If we know the approximate area where we should be, we can identify the most characteristic elements around us (mountains, rivers, villages...), locate them on the map, and deduce our position.

To orient a map we can use two procedures. The first one, if we have located ourselves on the map from the observation of the environment, is to place the map so that it coincides with our surroundings. It is widely used to orient street maps. Once oriented we can know the direction that we must take, the course, with only knowing to what point of the map we want to arrive. The course that marks the map is the same that we must take in reality.

However, sometimes we do not have these aids, for example if we are in a closed room or we are caught in the fog in the mountains, and to orient the map we need the compass. In a compass we must distinguish two important parts: the magnetic needle, which always points to the magnetic north, and the limbo which is the wheel where the degrees of the circumference and the north are marked.

As general norm in the maps the north is in the superior part of the sheet, the west to the left, the south below and the east to the right. If this is not the case, the map will have a compass rose which is a representation of the four cardinal points and indicates which is north.

To orient the map we place the compass parallel to the meridians, or the right or left edge of the sheet if there are no meridians drawn. Then we turn the sheet until the compass circle coincides with the direction marked by the needle. At that moment we have the map oriented.

The bearing is the direction in a straight line from one point to another, measured in degrees of circumference with respect to the direction between the same point and a third one. Generally the reference point is North, and the point from which it is measured is the point at which we are. In this way, to walk towards the east will be to follow a course of 90º: from where we are we draw a line towards the east, which is our course, and another towards the north, which is the reference, so that both lines form 90º. To know the degrees of the course between two points it is enough to use a protractor, although in practice the compass is used. It starts counting from the North and clockwise.

There are three types of north:

• The geographic or true north, which is the point of intersection between the Earth's axis of rotation and its surface.
• The magnetic north, which is the one indicated by the compass. The north indicated by the compass is not the geographic one although they are relatively close. The difference is called magnetic declination and its value depends on where we are located and varies with time. The professional maps indicate which is the value of the magnetic declination for the center of the sheet, and which is its annual variation.
• The third north is the one indicated on the map. In most maps the north is not a point but the whole top line.

The difference in the center of the sheet, in maps with UTM projection, between these three types of north is very small.

The difference between geographic and magnetic north was detected by Columbus, but it was not until 1831 that the magnetic north pole was found. This point can be located because in addition to the magnetic declination there is also the magnetic tilt, which marks the center of the Earth. It is zero at the equator and 90º at the magnetic pole.

There is a last way to know the bearing in practice without the need to orient the map. Compasses usually have a straight side and a movable limb. We place the straight side between the place where we are and the place where we want to go, with the back side at the place where we are. We rotate the limb until it is parallel to the meridians and pointing north on the map. We take the compass in our hand and turn it until the magnetic needle coincides with the north we have marked. Then the straight side of the compass will indicate the direction we should follow.

Elaboration

Nowadays topographic maps are elaborated from the interpretation of aerial photographs and other techniques of remote sensing, although formerly they were elaborated from manual measurements.

In addition to the classic topographic maps on paper, the rise of new technologies has given a predominant place to the digital maps (es), due to the possibilities offered to integrate and superimpose different maps, download routes and their integration with GPS systems (es).

Nowadays, map making is a complex operation involving groups of more than 50 different disciplines: photonavigators, mechanics, laboratory chemists, geodesists, mathematicians, topographers, geologists, biologists, geographers, physicists, agronomists, soil scientists, civil engineers, economists and architects, among others.

The science that deals with the study and preparation of maps is cartography and the profession, therefore, cartographer.

Types of topographic maps

Although the term topographic map covers all maps that provide information about the terrain in the manner already explained, it is important to know that not all topographic maps are suitable for going into the mountains. The maps of the Instituto Geográfico Nacional, for example, include all the elements of orography and terrain, but do not include in great detail important trails such as grand tours (es), which are very valuable for mountaineers.

In general, we can find official topographic maps produced by the administration and specialized maps produced by publishers.

Another possible categorization, of course, are paper topographic maps and digital topographic maps, which are already widely used.

National Topographic Map of Spain

The National Topographic Map of Spain is a set of cartographic publications produced by the National Geographic Institute of Spain (IGN). It is composed of six series of topographic maps at different scales: 1:25,000, 1:50,000, 1:200,000, 1:500,000, 1:1,000,000 and 1:2,000,000, covering the entire Spanish state. This topographic cartography is the base for the thematic topography produced by the IGN; and the series 1:25.000 and 1:50.000 conform the official basic cartography of the state.

From this cartography it is possible to obtain data on relief, hydrography, land use (natural resources, population centers, infrastructures), administrative divisions, toponymy, etc.

The six series that make up the National Topographic Map are:

• Series 1:25,000: Currently the basic and main series, and is the origin of the other series. It is formed by 4,123 sheets, each one representing about 12,500 ha. It uses the UTM projection. The formation of the current MTN25 series began in 1975 and was conceived as a complementary series to the MTN50 and limited to areas of special interest: coastal periphery, border areas and large urban centers. From 1980 onwards, it came to be considered as a series of total coverage of the State whose production was carried out using classic techniques. From 1985 onwards, it was computerized, culminating this process in 1994 and converting the MTN25 into a totally digital series.
• Series 1:50,000: It is made up of 1,125 sheets, each one representing some 50,000 ha. Historically it has been the basic series, although currently the IGN focuses more on the 1:25,000 series, and stopped updating this series. The new production of this series will come from the 1:25,000 digital series. In 1968 the 1:50,000 series was completed, coinciding in time with the beginnings of the new MTN25 series, which would eventually become the basic series of official cartography in Spain. In 1985, its updating by traditional procedures was halted in order to concentrate efforts on the production of the then new National Topographic Map at a scale of 1:25,000, MTN25, whose digital production greatly facilitated the generalization processes. As a result, in 1999 the design of the new digital MTN50 at a scale of 1:50,000 was undertaken, the first sheet printed being that of Aranjuez (605) in 1999.
• Series 1:200,000: This was the series chosen for the provincial cartography. It is composed of 48 maps, one for each Spanish province (the three Basque provinces are represented on a single sheet).
• Series 1:500,000: It was the one used in the National Atlas of Spain of 1965. The Lambert projection was used. Formed by fifteen sheets.
• Series 1:1,000,000 and 1:2,000,000: These scale sizes allow the entire representation of Spain. They are also used in some European cartographic projects.

From all the cartography offered by the IGN the 25 and 50 series stand out. From this page it is possible to zoom in on an area of the country to see how the whole geography is divided in the 1:50,000 scale maps, being able to see its sheet number. By accessing one of the maps, you can see more detailed information about this map (such as the year of edition) and the four sub-maps, at a scale of 1:25,000, into which it is divided.

On one sheet of the map the following information is shown:

• At the top the map name which is the most important population found within the map.
• In the upper right part appears the map number, and under it, between brackets, appear two numbers separated by a hyphen that indicate the coordinates of the UTM zone.
• In the lower left part (formerly in the upper right part) the numbers of the contiguous maps are shown. The numbers of the previous and next maps are correlative but the upper and lower ones are not. With UTM coordinates this is unnecessary since the first digit corresponds to correlative numbers horizontally and the second to correlative numbers vertically.
• At the bottom of the map appears the scale (both numerically and graphically) the value of the equidistance (20 meters in the scale 1:50,000 and 10 meters in the scale 1:25,000).
• In addition, the legend must appear indicating what the conventional signs mean, and the types of soil. In the modern maps a small box appears the administrative division. It also appears the date of the measurements. In some cases the value of the magnetic declination appears, that is to say, the difference between the north that marks the compass and the geographic north.

The National Topographic Map uses the UTM projection (es), which divides the world in spindles, although in the old maps the conic projection was used. To make the first sheets, in the 19th century, the geodetic network was used. The geodetic vertices are pivots with a square base and cylindrical termination located in various places, such as mountain tops, although church towers also serve as geodetic vertices. The latitude, longitude and altitude of the geodesic vertices are known with certainty.

Nowadays, maps are drawn on the basis of data provided by satellites and aerial photography. The geodetic vertices are located on them and from there the entire map is drawn. The old maps take as zero meridian the one that passes through Madrid, but the current ones take as zero meridian the Greenwich meridian.

IGN Viewer

The IGN has a free on-line viewer that allows to see all the Spanish geography, and depending on the zoom level shows the map at 1:1,000,000, 1:200,000 or 1:25,000 scale. It is also possible to view other layers, such as satellite images taken at different heights or information on land cover..

Among the most noteworthy features of the VisorIGN are:

• Topographic maps at 1:1,000,000, 1:200,000 and 1:25,000 scales.
• Satellite images.
• Possibility of downloading georeferenced images with a maximum size of 5,000x5,000 pixels, in jpg format with information in jgw format (not compatible with the CompeGPS program).
• Search by any name on the map (peaks, lakes, municipalities, plains...) or coordinates.
• Possibility of printing maps.
• Distance and surface meter.

In addition to current maps, the IGN has a wealth of information including a map library (es) with maps both before and after 1900, where you can search for the information requested by different criteria. Its purpose is to testify and preserve the different cartographic productions of the Institute.

National Center for Geographic Information

The IGN's information is accessible through the National Center for Geographic Information.

Mountain maps in paper and digital format

The page Mountain maps (es) explains how to obtain both paper and digital maps, highlighting the National Topographic Map (es) which can be obtained free of charge.

Tools

• Convert coordinates between decimal and degrees-minutes-seconds systems.

Bibliography

• The national topographic map of Spain (Web Archive) (es).
• Topographic map - Wikipedia (es) Topographic map - Wikipedia (es).
• Instituto Geográfico Nacional de España (National Geographic Institute of Spain) (es)]
• El mapa topográfico (es)