Natural shades aerial-photo or satellite maps

Abstraction

This survey examines possible ways foremost to keep or even extinguish the “false” sunglassess of the alleviation, created chiefly by the way of the natural sunshine presented on aerial-photo or orbiter images, and so to replace them with the “correct” unreal hill-shading sunglassess harmonizing to the cartographic rules of discernability and perceptibility, which allow the map user to easy and clearly construe the alleviation ‘s forms and formations. The natural sunglassess are eliminated from the aerial-photo or orbiter images by using particular radiometric and statistical processing in order to make images free of sunglassess. An analytical description of the Earth ‘s surface -stored as a digital lift model- combined with calculating tools of a assortment of hill-shading methods are used to bring forth a cartographic “correct” image of sunglassess of the survey country. Finally, image-processing techniques are applied to make new images composed by the “correct” sunglassess and the shaded free aerial-photo or orbiter images. The results indicate the potency of the integrated methods and applied techniques in order to build aerial-photo or orbiter maps clearly and decipherably perceived.

KEY-WORDS: Aerial-photo/satellite maps, alleviation visual image, hill-shading, light theoretical accounts, rectification of topographic consequence.

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Introduction

The established map definitions are by and large based on the graphical representation of the existent universe and the intrinsic phenomena or spacial relationships, where images and exposures are non conventionally parts of graphical symbolisation. However, the assorted definitions introduced through the clip, show that of import alterations have been accomplished in all function undertakings and processs, -mainly due to technological development- and lead to re-examination of the effectivity of bing cartographic regulations and rules [ Nakos and Filippakopoulou 1993 ] . In some of the proposed definitions, the content and usage of map are related to a wider scope of activities, affecting any in writing image that may include a spacial constituent, and subscribing to mapmaking ‘s evident involvement for new merchandises [ Kraak 1989 ] .In most of the instances, images of the Earth ‘s surface have been used for cartographic intents in an indirect manner, as beginnings of roll uping informations through photogrammetric and remote feeling techniques.

Late progresss in the usage of aerial-photo or orbiter images have come to add new capablenesss. These are:

  • The uninterrupted development of the Earth observation systems and detectors used for informations acquisition, by the agencies of increasing declaration.
  • The increasing handiness of the informations produced by these systems even to the simple users.
  • The attendant release of the appropriate package that supports the tools to take advantage of the new merchandises.

The potency of these new capablenesss enable users involved in geo-informatics to easy use digital function processs and bring forth graphical composings uniting aerial-photo or orbiter images with bing vector cartographic informations. This use of Earth ‘s images can offer a strong cue of acquaintance to map users, exciting their cognitive attack to back up cartographic perceptual experience. Map design and production may be challenged by the geographic expedition of the possible adjusting theoretical cartographic rules on the complex background of aerial-photo and orbiter function.

One of the most critical constituents of these new cartographic applications is the representation of the topographic alleviation, which is already an congenital feature of the image, caused by the natural sunglassess of the alleviation created from the incident sunshine. In several fortunes, the produced realistic sunglassess make the Earth ‘s topography reciprocally perceived ; whether the natural sunshine is directed E or even southeast. Cartographic methods of hill-shading usage analytical techniques of calculating alleviation shaded images characterized by a clear and legible perceptual experience of the Earth ‘s morphology. Such shadings could be added as a addendum to the original images [ Yoeli 1965 ] . However the ocular consequence will be improved if the portion of alleviation ‘s influence is isolated and removed from the existent tones, generated non merely from the alleviation but besides from assorted other factors as the flora, the Earth texture, the conditions, the conditions of sunshine, etc.

Elimination of the natural sunglassess in aerial-photo and orbiter images

The natural sunglassess of the alleviation present in aerial-photo and orbiter images are non ever created with proper light geometry such as the basic regulations of cartographic hill-shading suggest, in order to accomplish a right reading of the topography. In fact, the proficient agenda of orbiters ‘ capturing process is normally made to guarantee that a clear image of the natural surface of the Earth will be acquired, and this means that it occurs early in the forenoon when the Sun comes from the E. Aerial-photos ‘ capturing is easier to be programmed, and it normally occurs closer to the midday to avoid heavy dramatis personae shadows that conceal surface information.

However, in the above illuminating conditions still some shading does happen, and in add-on there are exclusions from the clip agenda which cause intense shading. So, if the Sun is low plenty, so the “fault” shading and shadows are severally heavy and they must foremost be omitted before using the computed “correct” 1s. Shading is a portion of the full physical procedure of light and glow recording, on which remote-sensed informations capturing is based. More specifically, the recorded glow values of Earth ‘s surface could ideally match to the nonsubjective measures of visible radiation reflected by the land, depending merely on the energy features of the land-cover. In pattern, there are two chief grounds impacting the concluding recorded values: the atmospheric fading caused by dispersing and soaking up, and the geometric consequence of topography on land coefficient of reflection procedure [ Krauss 1991, Jensen 1996 ] . The alteration in glow due to atmospheric fading is a really complex phenomenon, that several alternate appraisals have been suggested for its description. Although its influence on the whole procedure differs from the instance of orbiter to air-born capturing, in both instances its ocular impact works in a planetary manner.

In the context of the present survey, attending is focused on the topographic consequence, which is the chief ground for the formation of the locally dependent ocular feelings of shadowing and shadows.Topographical standardization is a basic radiometric rectification, happening in the preprocessing of remotely sensed images, and its computation is carried out by extended surveies. The proposed rectification methods refer to either an incorporate theoretical account for atmospheric corrections, decidedly necessitating land control measurings and cognition of detecting conditions ‘ inside informations, or to more practical mathematical and/or empirical theoretical accounts based entirely on the topography of the surface [ Karathanassi et Al. 2000 ] . Below are presented two normally used mathematical methods, the instead simple cosine rectification and a more effectual one called the Minnaert rectification:As it is obvious from the latter equation for k=1 the Minnaert rectification degenerates to cosine rectification. Evaluations of the known mathematical or empirical methods provide of import information for their effectivity [ Meyer et Al.

1993 ] . The cosine rectification, which is entirely based on the premise of a Lambertian contemplation of an isotropic distributed light by the lighted surface, tends to overrate shading in rugged terrain with decrepit illuminated countries, therefore it is chiefly applied in level terrain. On the other manus, the Minnaert rectification takes besides into history the constituent of visible radiation caused from diffusion and environing contemplation, by using a invariable that implies the extent to which a surface is Lambertian. Furthermore, merely little difference in consequences is ascertained between the Minnaert rectification and other complex statistical methods present in literature. Therefore, in the present survey the above two methods were used for practical grounds.In this work, two samples of Earth ‘s surface images, one aerial-orthophoto and one orbiter image, are used for an experimental process of shadowing rectification as large-scale or small-scale representations severally ( Figure 1 ) .

The angles of sunshine for the orbiter sample were found by the metadata information of older scenes of the same day of the month, since the scene used came without this information. For the aerial-photo sample, the clip and day of the month of the shooting were used as input informations to cipher the polar co-ordinates of the sun place. The several digital lift theoretical accounts were interpolated utilizing gathered height information, by agencies of either digitized hypsometric contours and breaklines from maps of graduated table 1:50,000 ( instance of orbiter informations ) , or single topographic point highs captured from stereo-pair images ( instance of the aerial-photograph ) .

At this point a critical issue arises mentioning to the matching between the digital lift theoretical account and the image to be corrected.Figure 1: Sample imagination and comparative available information for the corrections

  1. 3D visual images of the samples ‘ countries with draping of hypsometric shades over the interpolated DEM
  2. The several samples of orbiter and aerial-photo imagination
  3. Computed shading images matching to the place of the Sun at the day of the month and clip of informations capturing

Both methods of rectifying topographic effects are applied on the two samples, where the Minnaert invariable ( K ) is approached by consecutive estimates of values between from 0 and 1. The adoptive value of K was through empirical observation estimated by choosing visually the most effectual consequence. The concluding image values ( LH ) calculated by multiplying the corrections with the existent values ( LT ) , are normalized to the scope of grayscale visual image. Evaluating visually the consequences of the calculations applied on the orbiter image sample ( Figures 2a, B ) , it appears that the cosine rectification so causes overestimates. Contrary, the Minnaert expression applied with a changeless valued equal to 0.

6, gives satisfactory remotion of sunglassess with the attendant loss of 3-dimensional feeling.The instance of the aerial-photo ( Figures 2c, vitamin D ) has certain troubles. First, the alleviation matching to images of big graduated table is really elaborate and complex. The inclines at locations with unsmooth terrain are larger that the expected physical 1s, ensuing in highly high values of corrections at the equal locations. Assuming that the maximal logical incline on the land is up to 50 % , the maximal rectification is computed for the alleviation of this country, and values larger than this are ignored. The aerial-photo ‘s corrections so, go similar to the 1s computed for the orbiter sample. In this manner, the job of overestimate utilizing cosine rectification is non evident and this method produces more satisfactory consequences than Minnaert ‘s. An even more inconvenient job is that the shading tones interfere with tones of different contemplation due to land-cover, whose fluctuation is surely higher.

This deficiency of 3-dimensional feeling of natural shadowing in aerial-photos, due to the covering and dotted texture of Earth ‘s surface had been identified long ago, while shoping ways to pattern the early attempts on manual adept hill-shading [ Imhof 1982 ] . Still, it is clear to be seen that sunglassess of alleviation have been omitted, if the corrected image is closely examined together with the fake image of the alleviation shadowing harmonizing to the day of the month and clip of capturing.Figure 2: Grayscale visual images of corrections ( “the larger the lighter” ) and several consequences

  1. Cosine corrections on the orbiter sample
  2. Minnaert corrections ( k=0.6 ) on the orbiter sample
  3. Cosine corrections on the aerial-photo sample
  4. Minnaert corrections ( k=0.

    5 ) on the aerial-photo sample

Relief-shading for combined visual image with other surficial representations

The function of relief representation as a map constituent, varies from being a auxiliary background that enhances the word picture of the chief depicted phenomena [ Wheate 1996 ] to “the foundation for all the staying contents of the map” [ Imhof 1982, p. v ] , depending on the intent and type of the map. In any instance, the uninterrupted 3-dimensional nature of alleviation makes the pick of its symbolisation a peculiarly complicated and special-treated map process [ Robinson et Al. 1995 ] . One of the earliest developed methods for alleviation representation is the method of hill-shading, which gives a realistic and easy perceived word picture of the alleviation even for insouciant users. However, map makers had to wait for modern-day technological accomplishments in order to get the better of the troubles in production, and expeditiously pattern and experiment with hill-shading images.

The cartographic method of relief-shading or hill-shading is the ocular distinction of tone under specific light conditions and due to the variable local orientation on each point of Earth ‘s surface, counted with the per centums of the reflected visible radiation. In world the interplay of light with a surface is expressed utilizing many parametric quantities which denote more complicated physical theoretical accounts and position-dependent effects of light, like dramatis personae shadows, light from neighbour surface points or atmospheric position. But as cartographic hill-shading is used for stand foring and non imitating world, effects like these are non implemented because they do non needfully function the perceptual experience of the surface shapes [ Imhof 1982, Horn 1982 ] . Alternatively of that, convenient premises that guarantee legible shading images are considered ; Lambertian, absolutely spreading surfaces that demand comparatively simple computations, and conjectural sunshine that psychological grounds enforce its arrangement to west waies are applied. Further guidelines are the adjustment of chief light way to the dominant facets and inclines of alleviation, and accommodation of light beginning harmonizing to local surface orientation. Among the several methods of stand foring terrain information, hill-shading applies for its ability to show a quick, comprehensive, non-skilled and familiar image of topography. The aforesaid features make it the most proper method to visualise terrain information in combination with other phenomena that are besides portrayed by utilizing surficial symbolisation, eg.

polygonal thematic informations, discrete-zoned or uninterrupted realistic hypsometric shades, conventional parallel maps which have been scanned and geo-referenced, or images of Earth ‘s surface.The production of a relief-shading bed for combined surficial representation has two chief stages of processing, where certain picks can specify the concluding consequence. First and foremost, is the chief production of the hill-shading tones, necessitating a digital lift theoretical account of the country, and the choice of the most convenient available shadowing theoretical account and the appropriate parametric quantities. The common parametric quantities for any theoretical account are the AZ and zenith angle of the conjectural sunlight way -usually placed to north-east and 50 % from the ground- while particular other parametric quantities for a certain theoretical account might besides be.

It is really of import to observe here that, despite of the important advancement of research about hill-shading, merely a restricted scope of certain theoretical accounts can be found in the computational environments of the commercial and staple package merchandises used for mapmaking. If a wider pick of choice is needed, either usage of other type of package must be developed ( eg. based on 3D computing machine artworks ) or scheduling of modus operandis should be implemented along with the necessary tools for data-exchange. Harmonizing to the type of combined spacial representation, some general -even largely empirical so far- facets of taking the appropriate sort of theoretical account may be applied, but still it is merely the visual image of shadowing that can confirm if the pick of parametric quantities of shading is equal or non.Secondarily, the produced grayscale image of shadowing is farther adjusted towards its tonic features and texture, harmonizing to the combined spacial representation. The basic construct of this fine-tuning is to restrict the shadowing values to a subset of the full scope covering from black to white. The accommodations that can be made consist of the betterment of brightness and contrast, and the smoothing of imperfectnesss or unneeded inside informations utilizing appropriate filters.

At big graduated tables shadowing purposes to stress the dominant signifiers of the topography while at smaller graduated tables more elaborate information is needed, and the maximal strength is limited by the relationship between the shading and the discernability of map [ Keates 1989 ] . Successful combination of the constituent representations is achieved when their characteristics as they are presented in the resulted image are supported by the balanced interplay of each constituent ‘s part.As these alterations of hill-shading tones depend wholly on the combined surficial representation, they should be accomplished together with the concluding execution of two several images.

During the process of combination, there is one more manner to optimise the ocular consequence. By utilizing a regulative factor a, there can be a balance between the to the full assorted image and the initial one [ Tzelepis 2000 ] as described by the undermentioned equation:This is a characteristic manner of image combination, available in most of the image-processing package. This characteristic together with the ability of direct previewing, are really helpful tools for the consecutive execution of a hill-shading method.

Combination of computed hill-shading and a non-shaded image of Earth ‘s surface

A successful incorporation of relief-shading representation in a orbiter or an aerial-photo image free of sunglassess is achieved when the critical characteristics of the two constituents are preserved, or even further enhanced. A cardinal attack to this attempt is an apprehension of the specialnesss, carried out by the close scrutiny of types and operation of the ocular elements for each constituent. As indicated by a conceptual model of image analysis, tone or colour is a cardinal belongings of imagination, while other ocular elements like form, form or texture are expressed by agencies of spacial agreements of it, consecutive perceived in an intermediate or higher degree [ Estes et Al. 1983 ] . All elements are working in the context of several image analysis undertakings ( sensing, designation, measuring, knowledge-assisted labeling and significance ) , for taking at the reading and perceptual experience of the spacial objects and at relationships among them. In hill-shading images a similar state of affairs is evolved ; tone is the cardinal belongings, forms and forms are formed by agreements of tone, uncovering the signifiers of alleviation in the presented country. The ocular component of size is besides evident in both constituents, but it functions with more item in the context of the imagination. By and large, these two visual images seem to be perceived in a similar manner, subscribing to their combination for bring forthing harmonic ocular consequence -here carried out by generation of the several images ‘ values at matching points.

Figure 3: Computed hill-shading images and combinations with the orbiter imagination sample.

  1. Standard theoretical account of Lambertian surface based on the perfect diffusion premise. The sunlight AZ selected for better shading is 156A° NE.
  2. Local accommodation of AZ and lift of light way for better lightning of the country, comparatively to the consequence of 3a ( here equal to 25 % for each angle ) .
  3. Fine-tuning of shadowing on illustration 3b, by tonic alterations ( increased brightness 50 % and contrast 25 % ) .
  4. An alternate pick with likewise balanced consequences, produced by a simple mathematical theoretical account ( parametric quantities to be defined are the grey tone for horizontal surfaces and the rate of alteration for the grey tone to incline ratio, here equal to 50 % each, brightness and contrast of shadowing image have been improved ) .

The presence of hill-shading adds to the image the indispensable cue for depth perceptual experience, which is required in order to run as a non-perspective simulation of three-dimensions [ MacEachren 1995 ] . In contrary to the original captured images, the alleviation signifiers perceived utilizing proper shading regulations correspond to the existent 1s. Furthermore, for those spacial objects that are shaped along with implicit in alleviation formations, it can be seen that perceptual experience is imposingly enhanced. In large-scale images ( aerial-photos or high declaration satellite imagination ) objects like these can be unreal characteristics like route web or transitions around edifices. In this instance of class these objects must be besides included in the utilised digital terrain theoretical account. The good influence of stand foring the alleviation with right hill-shading can be seen in all phases of image analysis ; from the early phase of sensing where mental images are recalled to assist the perceiver to label the object, to more time-consuming consideration needed for more complex agreements, where particular related cognition might be required. In the contrary, there are instances where the perceptual experience of alleviation is helped by locative information derived from the image, remembering existing particular cognition related to the site.

In fact, the reading of objects and spacial relationships presented in the images of Earth ‘s surface, and the perceptual experience of signifiers of alleviation accentuated under conditions of right shading, are reciprocally enhanced.The corrected samples utilized for this work, are combined with hill-shading images computed from the staple theoretical account of Lambertian contemplation ( Figures 3a, 4a ) or other alternate methods. More specifically two methods are tested for the satellite image ; the accommodation of light beginning for better local contemplation and brighter sunglassess ( Figure 3b ) and a simple mathematical method based on equilibrating the consequence around a defined grey tone for horizontal surfaces ( Figure 3d ) . For the aerial-photo the same theoretical account was applied, delegating the horizontal surfaces with a darker tone ( Figure 4b ) .

An illustration is given besides utilizing a method of mirrorlike contemplation designed for usage on computing machine artworks, which gives an image of “plastic” shadowing that fits the realistic feeling caused by detecting in a close distance ( Figure 4d ) . The dominant facets of alleviation are found, bespeaking convenient waies for conjectural visible radiation beginnings. Further alterations of brightness and contrast are made to optimise the shading towards to integrate it into the imagination ( Figures 3c, 4c ) .

The tuning of the shadowing image in order to uncover the alleviation signifiers consists of delicate and critical graphical determinations taken under serious restraints. The fact, that slight alterations of shadowing are impossible to be seen in the uninterrupted presented information of the images, suggests that enforcement of contrast should be high plenty to allow the sensing of alleviation forms. On the other manus particular attention is needed to avoid the presence of dark tones upseting the discernability and readability of imagination ‘s information, which is still the first precedence of this combination.Figure 4: Computed hill-shading images and combinations with the aerial-photo imagination sample.

  1. Standard theoretical account of Lambertian surface based on the perfect diffusion premise
  2. Use of a mathematical theoretical account for more balanced consequences, because of the dark tones caused by being of steep inclines ( same parametric quantities to be defined as in [ Figure 3d ] , with the grey tone for horizontal surfaces here equal to 12.

    5 % and the rate of alteration for the grey tone to incline ratio, equal to 50 % )

  3. Fine-tuning of old shading by tonic alterations ( with increased brightness % and contrast 25 % )
  4. A different pick produced here by a mix of 50 % Lambertian and 50 % mirrorlike contemplation for computing machine artworks ( brightness and contrast of shadowing image have been improved )

Reasoning Remarks

At the context of this work, an effort has been made to experiment with optimum visual images of the relief-shading in the new fast-spreading orbiter and aerial-photo function merchandises. For practical grounds of simplifying the process at this first attack, merely grayscale images were used. The several undertakings of the process were carried out utilizing commercial package for GIS and conventional image processing ( ARC/Info, Adobe PhotoShop ) and custom scheduling modus operandis. First, existing natural sunglassess that mystify the perceptual experience of users were removed utilizing techniques of radiometric pre-processing.

The computation of corrections from direct contemplation by these techniques, based on the Lambertian contemplation jurisprudence, is merely a portion of an incorporate process for environmental noise. The inclusion of more parametric quantities like contemplation from neighbor countries or sprinkling can be farther examined for more dependable calculation of rectification.The complexness of the imagination ‘s information itself requires that the production of the hill-shading representation demands to be implemented along with the concluding undertaking of its incorporation in the image, in order to avoid perturbations on image reading and concluding merchandise discernability. Strong ocular tools like direct previewing are indispensable for acquiring out the most of this delicate graphical process. This demand becomes even more of import if colour images are to be used, presenting another critical point of the choice among alternate combine processs based on different colour theoretical accounts or graphical techniques. The attempts described herein constitute some general experimentation based on the features of the individual constituent visual images. For an nonsubjective certification on the perceptibility of the combinations of hill-shading methods and the images, it is suggested that related questionnaire including ocular illustrations like the above, should be planned and applied on users, so that information for statistical processing to be provided.

In add-on, it is a strong challenge to lucubrate these first consequences in order to develop the full undertaking to 3-dimensional visual image.

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