Access to Landsat DataAuthor(s): R. J. Gurney, R. F. Templeman and Ray HarrisSource: Area, Vol. 11, No. 3 (1979), pp. 223-225Published by: The Royal Geographical Society (with the Institute of British Geographers)Stable URL: http://www.jstor.org/stable/20001469 .

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Comment 223

McEvoy's view that all segregation is due to racial discrimination' (our italics). At no point did we portray racial minority space in these crude terms: our view is that

minority residential patterns result from an interaction between ' internal ' forces (the needs, desires, aspirations and choices of the minority itself) and 'external ' forces (limits imposed by the majority in society). We suggest that in a white-dominated society, whose entire modern history as a colonial power has been bound up with the exploitation of non-white peoples, the external half of this relationship will be dominant. This being so, the bias towards minority behaviour which we detect in much of the British geographical literature on race is unjustifiable and ought to be corrected. This is very far from a bald statement that minority residential behaviour is totally unimpor tant or that self-segregation does not occur in Britain: it is merely a plea for a shift in emphasis and a recognition that racism is a dominant rather than a secondary considera tion. Let us charitably assume that Peach's assault on us stems from genuine misunder standing of our position and hope that our clarification will bring this geographical version of ' The Mousetrap' to a close.

References Cater, J. C. and Jones, T. P. (1979) ' Ethnic residential space: the case of Asians in Brad

ford', Tijdschr. econ. soc. Geogr. 70, 86-97 Lee, T. R. (1978) 'Race, space and scale', Area 10, 365-7 McEvoy, D. (1978) 'The segregation of Asian immigrants in Glasgow: a note', Scott.

geogr. Mag. 94, 180-2

8. Access to Landsat data

R. J. Gurney and R. F. Templeman (Institute of Hydrology) write:

In promoting the use of digital satellite data sets, Harris (1979) gives a short FORTRAN computer program segment to read Landsat computer compatible tapes. This segment, however, is not of general applicability because it neglects a number of matters. First, implementations of FORTRAN on different computers and even with different compilers on the same computer often lead to differing layouts of information being expected by a FORTRAN read of a file written on a magnetic tape: a FORTRAN read will often expect headers, trailers, checksums and record blocking information to be interspersed within the record data. Secondly, some computers are word machines, and some are byte machines, which will handle LOGICAL*1 and INTEGER*2 declarations quite differently, and the number of bits per word on word machines will affect the way numbers are stored. Thirdly, different computers may use different internal codes to represent alphanumeric information. Hence it is not just a question of improvement, but a necessity, to use local routines to read the tapes on many computers; in addition, byte manipulation and character conversion routines will be necessary to make sense of and use the data successfully.

An approach which may be adopted can be illustrated by an outline of the tape translation programs for satellite data used at the Institute of Hydrology on the

NERC Univac 1108 computer, which is a word (of 36 bits) machine. The data are read, block by block, using a Univac 1100 assembler subroutine called from a FORTRAN program. The number of words in each block is passed to the subroutine, which returns the status of the read attempt (indicating end-of-file and various error conditions) and an array of integer data. The data are read so that each 8-bit character on the tape is put into a 9-bit integer quarter word on the Univac. This reduces storage,

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224 Comment

because data from four pixels can be stored in each Univac word. The radiance values for each pixel are retrieved as desired by using a set of functions written in Univac 1100 assembler to extract the relevant quarter word from the array of data and placing it right-justified in a whole word, zero-filling the rest of the word. Data for the headers are treated similarly, except that alphanumeric data in character code is blank filled rather than zero filled and, in certain cases in the header, half words rather than quarter words must be retrieved. In some cases an EBCDIC to ASCII character conversion is also performed.

This approach has been applied to satellite data in several tape formats which have been read on the Univac 1108 computer, the details depending on the exact formats of the tapes. Users should always refer to the relevant users' handbook to check tape formats and details of the original data. For instance, not all Landsat radiometric data are quantized to the same number of bits, even for different channels of the same scene. Landsat tape formats have also changed over time and between the sites from

which data are distributed. It is thus necessary for all reading programs to be modular to accommodate these changes.

It has been shown that the program segment given by Harris (1979) is only applicable on a limited range of computers that behave as Harris assumes. Harris should thus elucidate the conditions under which the program segment will give correct results.

Reference Harris, R. (1979) 'Access to Landsat data', Area 11, 63-6

Ray Harris (University of Durham) replies:

I agree with Gurney and Templeman that users of Landsat CCTs, or any other CCTs for that matter, should consult the relevant manuals for reading magnetic tapes, and be fully aware of the relevant specifications for handling magnetic tapes of a particular computer installation. The FORTRAN IV program segment given in Harris (1979) is a very simple routine used on the IBM 370/168 computer of the Northumbrian

Universities Multiple Access Computer (NUMAC) for reading Landsat CCTs which were written on a similar machine in the United States. NUMAC is available to users throughout the UK from remote terminals, where the user can substantiate a claim to have access to this particular machine. So, if reading CCTs presents a problem on local computer installations, then it may prove possible to have access to NUMAC.

Gurney and Templeman quite rightly stress the importance of referring to user manuals, particularly where data processing systems change over time. At the beginning of 1979, the EROS Data Center introduced changes to their processing facilities by introducing the EROS Digital Image Processing System (EDIPS), which will routinely process high density digital tapes of Landsat data to provide improved digital and film output products which have been radiometrically and geometrically corrected (NASA, 1978). Image masters will be produced at a size of 241 mm on a high resolution laser film recorder, and Landsat CCTs will be produced in a new form. The existing band interleaved by pixel pairs CCT format will be replaced by a choice of either band sequential (BSQ) or band interleaved by line (BIL) format. The BSQ format CCT has N separate files, each file containing annotation and image data for each of the N Landsat bands, while the BIL format CCT contains image data presented one line at a time for each of the N bands in sequence. Systematic corrections can be applied to the Landsat data at EDIPS to correct for (i) band to band offsets, (ii) dif fering line lengths, (iii) Earth rotation, and (iv) detector to detector sampling delay. Image data can also be geometrically corrected to one of three map projections: Space Oblique Mercator, Polar Stereographic, or Universal Transverse Mercator. The

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Comment 225

resampling of the Landsat data for these projections is performed by either nearest neighbour or cubic convolution routines. In addition, EDIPS also provides facilities for enhancement of imagery, and the production of summary statistics. Holkenbrink (1978) gives fuller details of the new CCT structures. These changes in data processing at the EROS Data Center will make it easier for Landsat products to be interpreted, and for the data to be used in conjunction with other geographical data referenced to standard projections.

References Harris, R. (1979) 'Access to Landsat data', Area 11, 63-6 Holkenbrink, P. F. (1978) Manual on characteristics of Landsat computer-compatible tapes

produced by the EROS Data Center Digital Image Processing System, version 0.0, United States Geological Survey, Sioux Falls, South Dakota

NASA (1978) Landsat data users' notes 3, United States Geological Survey, Sioux Falls, South Dakota

9. Drainage density - basin area relationship

Peter Vincent (University of Lancaster) and Jean Haworth (University of Salford) write:

In recent issues of Area there has been much heated debate on the nature of the relationship between total stream length (EL) and basin area (A) (Gardiner et al., 1977; Ferguson, 1978; Pethick, 1978). Much of the controversy is related to the values of the coefficients in the deterministic relationship X =L aAb. Sadly, little controversy has been aroused regarding the estimating procedure which in our view

might have fundamental implications for the statistical arguments in this debate. All the authors just cited estimate the coefficients a and b by linear regression after

logarithmic transformation. Thus the regression model becomes

log?L = a+ b log A (1)

or more explicitly (Ferguson, 1978)

logXL = a+b log A+e

where e is the stochastic disturbance term. In this note our purpose is not to quibble with the geomorphological arguments but

merely question the reliability of the coefficients when so little attention appears to have been paid to model specification.

The manner in which the stochastic relationship is specified is crucial to the method of estimation. In the present case we may hypothesize two forms of stochastic relation ship:

XL = f(A) + e (2) an additive disturbance

or

EL = f(A)e (3) a multiplicative disturbance.

Depending on which of these we believe to be correct, the distributional assumptions we make about e, and the nature off(A), we adopt certain computational procedures to estimate the population regression function. If the stochastic relationship between EL and A is defined with a multiplicative disturbance, that is, ?2L + aA be, then logarith

mic transformation yields log EL = a+ b logA + log e which is linear in a and b and suitable for solution by least squares. However, if the stochastic relationship is of the form ?L = aA b+ e then taking logarithms does not enable us to make the b linear

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