In: Csáti Ernő (szerk.): Hungarian
Cartographical Studies. Hungarian National Comitte, International
Cartographic Association, Budapest, 1989
The topographical survey of our Earth cannot be told even in the case of the continents as completed. Nevertheless if the demand on data for the small-scale maps are concerned the available information for the relief presentation is sufficient.
There is an other situation in the case of the sea and oceanic areas. More detailed survey (1:20,000—1:200,000) were carried out first of all for economic reasons: at coastal areas (harbours), on the shelves (exploitation of oil, natural gas or other mineral substances) and at the closer environs of islands, sandbanks (sailing).
At oceanic regions a more detailed survey was done only at areas, which were supposed to be interesting for any scientifical reasons. Here there can be put only in part the charts, which were made with the most recent method, the so-called Sea-Beam method, but essential part of them has not been published. The data of the oceanic bathymetric soundings for civil purposes are nowadays gathered on charts 1:1,000,000 (Plotting sheets) and they are published averagely in every 15 years as a series 1:10,000,000, resp. 1:6,000,000; (GEBCO = General Bathymetric Chart of the Oceans).
Over the direct surveying such methods are nowadays of great importance also for the mapping of the reliefs of the deep sea bottom, which provide only estimated data and give only in the case of fulfilment of certain criteria approximately good result (remote sensing—Seasat).
In view of our matter we can divide the contours in two groups the isohypses, which represent the relief of the continents and the isobaths, which express the depth of the sea bottom. The relief representation by contours is nowadays the most wide-spread method of the representation of the surface of the solid earth, respectively the basis of the bulk of other representation methods (e.g. hypsometric and bathymetric tint; hill shading etc:). The exact compilation of the contours—from the point of view of the relief representation—has decisive influence on the accuracy of the chart. But the smaller the scale of the chart to be edited, the fewer and "combed", smoothed contour draft has to reflect the picture real and well approaching the relief to be represented. The compiler of the chart can reach it through the laying down the rules of the generalization and—in the course of the map production—the consequent adoption of these.
The generalization is a subjective work and it remains—regretfully—for a long time as it is. Though we can define numerically some objects: e.g. in the case of reduction of scale how many per cent of the names, vallies or other details could be left approximately, but which should be these, it can be found no concrete answer. The reason of the subjective character of the generalization is the man itself, the limited knowledge of the map compiler, on the field of the earth sciences—respectively the cartographical, physical, geological, geophysical, morphological etc. knowledges!
The relief has vertical and horizontal components. The questions of the generalization can be devided in compliance with it in two parts:
The generalization in vertical sense means the definition of the contour values to be represented, i.e. the definition of the level surfaces chosen for representation. (The lines of intersection of the level surfaces chosen on this way with the real relief present the contours.) The generalization on horizontal sense means the definition of the run of the contours—like plane curves—on the level surfaces chosen to be representated, to be more exact, how can the lines drawn on the charts—according to which principles—deviate from the “real” curves. (In classical sense this is meant mainly under generalization of the relief: the developement of the precise sketching.)
In both cases the generalization will be fulfilled more or less through the subjective adoption of the rules, in order to be the relief representation of the chart nearly as good as real to the picture of the relief of the Earth within the limits of the scale.
The most of the authors point out the importance of the right generalization of the level surface series, which suits well the aim of the chart representation and follows well the relief of the area wanted to be representated. Let us present here a figure for better illustration of the question (Fig. 1.).
Perceptibly in the case of the illustration of small areas the possibility is given in the fact for the suitable choice of level surfaces which enable the “true” image of the reality. There is an other situation, if we undertake the representation of large areas (continents, oceans) in small scale or of the surface of the whole earth.
Eduard Imhof enumerates e.g. in “Kartographische Geländedarstellung” (Walter de Gruyter et Co., Berlin, 1965) different solutions of six kind for the representation of continental areas. Among these he thinks the best the level surface series in values growing in height, parallel to the geometrical progression compared with the sea level (Fig. 2). But he does not think a convenient solution the “steps of equal area” (Fig. 3). He mentions it only because of its theoretical interest, “while it calls the attention to the necessity of a more detailed division in flat areas.” Further on we shall demonstrate, that both these solutions lead to the same result by the convenient choice of parameters.
For the representation of the sea areas
Imhof publishes two figures. One of them illustrates the bathymetric steps
figuring the geographical maps at small scale (Fig. 4), the other one
demonstrates the bathymetric steps, level surfaces figuring on the large-scale
charts of the shelves (Fig. 5). After that he summarizes in table the level
surfaces suggested for the charts in different scales (Table I). Starting from
the conviction that the solid earth surface covered by sea water is comparable
as regards the geographic structure and diversification of the relief with the
continental one, we looked for the solution in order to represent uniform the
continental and, sea areas—which is the right one from the theoretical point of view at least. But in our
opinion the sea bottom is on our charts only for that reason unrelieved because
the level surfaces represented on these are sparse. In order to demonstrate it
we have compared the (terrestrial) Carpathian Basin with the (oceanic) Iberian
Basin (Figs. 6.a and 7.a) in such a way, that we have sketched first of all the
bathymetric contours at the Iberian basin which are forming exactly the same
level surface series in comparison to the datum level of the basin as usual at
the representation of the Carpathian basin (Table II).
Table I (after Imhof)
Nowhere the Carpathian Basin comes to 0 m, the Iberian Basin to –5500 m. These levels are the data surfaces of comparison. After these we have represented both charts with the contours adequate to 1000 m (Figs. 6.b and 7.b) and later 2000 m (Figs. 6.c and 7.c.) level surface intervals as usually at the representation of the oceanic areas. In the latter case the physical (basin) character of the Carpathian Basin became fully unrecognizable. That means: it is unfeasible neither at small scale maps to adopt 2000 m level surface intervals nor at the representation of deep-sea areas even! Therefore the task is to define such level surface series (theoretically at least), which results also at the oceanic areas a detailed picture similar to the continental one.
At the former figures after Imhof we have seen that generally the hypsographic curve (Fig. 8) is adopted in such comparison. But it must be mentioned that the curve in such form is not enough to draw any right conclusion. The function demonstrating the frequency of the height and depth values on the earth surface (Fig. 9), which can be easily demonstrated (see the broken line) that the distribution of the different depth in the oceanic areas does not change so simple as this of the continental heights. This suggests for itself, that the detailed representation of the sea bottom relief equivalent to the continental one needs much more contours.
The extremely strong mathematical approach of the level surfaces to be generalized isn't very practical, because the fractional values are hardly to be represented. Let us prove it therefore under less precise conditions: let the chosen level surface series such one, where each of the height or depth intervals have (nearly) equal areas under the frequency curve. In order to calculate easier, such data are needed, which demonstrate numerically, how many per cent of the whole earth surface belong to certain height and depth interval.
The data of the first three column of Table III are taken from "Der Meeresboden" of Eugen Seibold (Springer Verlag, Berlin, 1974).
Let us join to a rather small piece of earth surface (respectively right fitting to the data) let us say 3%, a level surface (see the principle of Imhof about the steps of equal area). ln this case the level surface figures being in 4th column of the Table proceed nearly to the different intervals. The last column of the Table contains tthe possible level values.
As to see, the method—“steps of equal area”—was adopted, which Imhof doesn't think convenient in the practice. And this had a similar result as the method he kept right for the continents. It is true, that the line growing according to the fully regular geometrical progression would be the following one: (0), 25, 50, 100, 200, 400, 800, 1600, 3200, 6400, but Imhof himself made “concession” in order to adopt the contour values having been current in the practice: (0), 200, 500, 1000, 2000, 4000 m.
The method adopted by us draws the attention much more to the necessity of the more detailed distribution within the flat areas, which is pointed out in connection with the method “steps of equal area” of Imhof. However this is valid for the representation of the oceanic areas too! And this will be emphasized also at the oceanic areas between –3000 and –6000 m.
It is evident, that for lack of appropriate detailed data the result yielded is only of theoretical interest. Nevertheless it is important from that point of view of supporting·it, that the sea bottom is only therefore unrelieved and flat at our charts because the represented level surfaces (contours) are sparse. At the same time it draws the attention to the princip1es, which are to be followed in order to present the sea bottom relief right if we have already sufficient depth soundings. On the other hand it points out too, that a sea bottom relief representation is unthinkable without sketched bathymetric contours (isobaths)—only by bathymetric layers, that means, that within each coloured “bathymetric layer” there must be adopted further auxiliary bathymetric contours with a view to the better expression of the relief formations. This conclusion is valid of course for the areas 0 to 200 m of the land, where we have got already the necessary data.
Such representation method must be adopted at our maps, which “prefer” the isohypses and isobaths used and accepted in the present general practice as boundaries of the layer tints, and at the same time it adopts, according to need, the contours belonging to the levels in the last column of the Table III as auxiliary contours too.
The rules of the quantitative and qualitative generalization of the run of the land relief contours, that of the coastline of the lakes and the rivers are worked well out. This can be told similarly in connection with the generalization of the oceanic and sea coastlines too. But while the principles of the generalization of the land relief, being in the possession of the structural, genetic, morphological knowledges—are spred widely, for the generalization of the relief of the sea bottom there doesn't yet exist satisfactory formed, unified rule. Several charts and map series bear evidence of it, e.g. the GEBCO too, at the 5th edition of which (Canadian Hydrographic Service, Ottawa, 1975—1982) the processing of the sheets is not uniform.
By the production of a good map the new knowledges of the earth sciences are absolutely necessary not only in the course of the map – making but also at the selection of the. right chart: references. At the forming of the sea bottom relief the endogenous (inner, tectonic) and exogenous (outer) forces being operative on the Earth are acting differently, with other dimensions and effects as at the land. It follows that the generalization rules of the land reliefs cannot be adopted automatically by the representation of the sea area.
The plate tectonics has disclosed the importance of the tectonic forces in the forming of the sea bottom relief—at the origin of the oceans at all. From the beginning of the 1960's light has been thrown step by step on the importance of the outer, exogenous forces, processes. Previously the basins of the seas and oceans—as the “last base level of erosion”, the regions lying under the sea level—were found in the literature as enormous zones of sedimentation of our Earth only. But the role of the outer forces cannot be even negligible at the forming of the reliefs under the sea level. At the coastal areas and the shelves the forces of tidal stream, the longshore currents, the prevailing climatic components (corals, resp. continental ice: glaciers, shelf ice); at the continental slope and the continental rise the turbidity currents from time to time; at the deep sea areas the deep sea currents moving from the polar region to the equatorial waters change the developed forms set up by tectonic forces and varied by different large sediment accumulation.
One of the possible approaches of the systematization of the sea bottom formations can be done on structural-morphological basis. In such sense we can speak about three big divisions:
a) continental margin
b) ocean-basin floor
c) mid-oceanic ridge
(Heezen, B.C.—Menard, H.W.: Topography of the Deep Sea Floor. in: the Sea (Vol. 3.) John Wiley and Sons, New York, 1963.).
a) The continental margins have two sorts in structural respect, we can distinguish the active and passive margin. To the active margin (Pacific type) a subduction will be linked—i.e. the slip of a lithosphere plate under the other one. It is characterized by a relative narrow shelf, a continental slope, which runs more bluff as average and an oceanic trench, wich “occupies” the place of the continental rise. It is interesting the structure of the Pacific, where the “mid-oceanic” ridge takes the position in general close to the American Continent (on some places he slips under it too!). Here at the margin of the continent the oceanic trenches run (subduction zones).
Far away on the other side of the ridge, not far off of Asia they are to be found on the east side of several island arcs.
The passive margin (Atlantic type) can be characterized by a wider shelf area, a continental slope of a fewer bluffness and well developed continental rise. Their basement, which originally formed continental crust, preserved the structure developed by rifting. (The rift is a topographic depression of regional or global extent formed by appreciable displacement along normal faults with roughly parallel strike, and associated with seismic and commonly volcanic activity.)
At the areas of continental margin there are characteristic the extensive accumulation of terrigenous sediments, the development of the relief formation is in connection with this accumulation and the sediment transportation.
b) The province of the oceanic trenches (at the continental margin of Pacific type) resp. the well developed continental rise— with the abyssal hills occuring from time to time—(at the border of the continental margin areas of Atlantic type), marks the transition to the deep sea basin lying in depth 4000—6000 m, which is formed by the range of basins divided off each other through positive abyssal relief forms. At the marginal parts of the abyssal basins towards the land—at the Atlantic type—is the accumulation of the terrestrial sediment characteristic too, which yields his place to the abyssal sediments step by step. But at the basins near to the continental margin of pacific type the oceanic trenches suspend the mass of the sediment of terrestriel origin, which mixed with the abyssal sediments transported by the oceanic plate is submerged in the subductional zone. (At the Asian coasts of the Pacific Ocean the “real” terrigenous sediments can not even reach to the open oceanic zones because of the island arcs. But for the sediments of terrestrial origin coming from the island arcs the fate mentioned above is in store too). At the areas of the abyssal basins towards to the mid-oceanic ridge the abyssal sediments predominate unanimously.
In
genetic, structural respect the province of the abyssal basins have oceanic
crust.
Within the abyssal basins, respectively at the adjoining territories there are to be found singular seamounts, seamount groups, seismically inactive ridges, rises and other positive and negative relief-formations.
c) The central provinces of the mid-oceanic ridges are the seats of the origin of new oceanic crust. Approaching to the axis of the ridge the sediments become thiner, but in the surroundings of the central rift valley there is practically not even considerable sediment accumulation. The differences appearing in the spreading rate are expressed in the structural-morphological forms too, thus in the sinking of the crustal part in different measure, resp. in the divergences of morphological appearance of the central rift valley (sometimes in fully absence) etc. At these provinces the primary relief forms effected by tectonic volcanic—let us say: ridge-forming—processes are dominant, which are refined, later on rather covered through the abyssal sediment only in larger distances in comparison to the ridge axis.
The limited volume of this study makes unable to discribe the smaller formations within each structural-morphological unit more detailed too. But the treatment of some question of essential importance—exactly in connection with the cartographical representation of the ridge area (one of the most critical province)—is needed.
The cartographical extrapolation and the generalization of the ridge-areas
By considering of the GEBCO-sheets it occurs quickly, at some areas of the oceans how few deep soundings were available to compile the bathymetric contours. The source lists of the sheets advert in many cases to it too, that other geological, geophysical source books were used too to define the isobaths.
At each sheet the areas were noticed too, where more detailed, larges-scale maps were at disposal. These topographic surveyings in very limited extension and relatively few samples in large scale come fortunately from the different areas representing the structural-morphological features of the oceans. By the use of these and other geological, geophysical data the relief sketching of fewer known, but in structural-morphological respect “related” areas can be refined with the help of cartographical extrapolation.
After “The Hungarian Explanatory Dictionary” the extrapolation is: the approximately definition of the values outside the territory of the studying through the generalization of the regularity of the experimental values.
The cartographical extrapolation is no other than the use of geological-geophysical knowledge and data on behalf of the uniform cartographical representation of the Oceans by isobaths on small scale charts. The conversion and refining of the bathymetric contours yielded through the regularely interpolation of the measured depth data, which results a better expressing representation of the geographical-morphological appearance of the undersea features. The main point is that it “projects” in respect to cartography the forms of territories surveyed in details on fewer developed territories on the basis of geological-geophysical common characteristics. The basis of the reason for the existence is the small scale: namely at these maps the primary aim is first of all the emphasizing of the morphological-structural characteristics, the secondary one the measurability. (If it would not be in this manner, a considerably part of the fjords could not be represented on our small scale maps—sometimes over extent). The adoptìon field of the cartographical extrapolation is limited to the ridge area and its nearer surrounding, because this is the area, where the accumulated abyssal sediment has not yet covered (or only in very small extent) the developing primary structural forms (but it did not cover them fully up and fundamentally has not converted them).
The practical method can be described—with knowledge of the exact region and the focal mechanism of the earthquakes—as follows:
The seismic activity at the mid-oceanic ridge system is limited to the central rift valley and the section of the transform faults (fractures cutting transversal the ridge) between the ridge-axes. It generalizes our experience affirmed by the large scale maps that the transform fault is a valley-like feature, we can draw a valley-form on the plain running isobaths yielded through the regular interpolation of the measured data at the place and direction of the transform faults demonstrated by geophysical measurement.
The mid-oceanic ridges are in genetic respect the places of the formation of the new sea bottom. In morphological respect the characteristic point of this “mountain system” is, that they are out in smaller parts through fracture zones (transform faults perpendicular (near) to the axis of the ridge. The most essential question of the generalization of the ridge areas is to keep these characteristic directions even at large scale generalization.
At our figures you can see the wrong (Fig. l0.c) and right (Fig. l0.b) solution of the generalization starting from a part of the sheet 5.12 of GEBCO (Fig. l0.a).
In connection with the representation of the ridge area the attention is drawn to an other very important symptom too: a definite difference is to be made between the ridge areas forming at the plate boundary of fast and slow spreading rate. The central rift valley appears definitely at the. areas with slow spreading and it must be represented also in very small scale too (Fig. 11) While at the territories of fast spreading rate (Fig. 12.) the ridges—in respect of the cross-section—have a smaller angle of slope and the rift valleys can not be represented in small scale at all. The molten mantle-material flows up from the inner part of the earth through thin fissures. Much more evident is the difference proved to the segments perpendicular to the ridge axis (Fig 13 and 14).
The studying of the reliefs of the small-scale maps, atlases, globes published in our country—but the analysis of several foreign maps, as well—has led to the result that the representation of the plain area on land is slipshod and schematic: and in the case of deep-sea areas the schematical representation is general.
The reason of this is that the generalization in vertical sense, i.e. the choice of the representated level surfaces (the contours) is not sufficient. For the representation of the relief in required details the density of the isolines (contours) on the small-scale maps—which was in the practice till now—is not sufficient at all. It is provable that on the land, but specially at the sea-bnottom (theoretically) there would be needed considerably much more isolines for the better representation of the relief-configurations. At the same time in the sea-areas it is necessary to work out the convenient rules of the generalization in horizontal sense (drafting accuracy of the isobaths), because some of the cartographers—failing the general rules and principles—make the generalization of the isobaths, which is already subjective enough, in very different ways and this results in a very considerable different representation of the relief.
Considering that in the case of maps based
exclusively on colouring of layers and bathimetric layers the number of the
"steps" to be representated is very limited, this kind of map is not
suitable for representation of the relief with the required details and of high
level. At the published mapworks we have found more precedents, where the relief-representation
of the map did not reflect at all the form belonging to the name of the
undersea feature.
As a solution
the opportunity occurs that we have to separate the “representative”
isoline-intervals (as layers) accepted in the practice by individual colours,
but we have to use inside of these layers further isohypses and isebaths (as
auxiliary isolines) in order to realize more detailed representation of the
Earth's solid surface.