49

me and patience

set of templets.

ROL METHOD

airphoto map

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in a map.

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ed to the basemap,

c information

s.

at, highly

rue to scale and

orientation. The question now is how true this map

really is. In the following Section the

theoretical and empirical accuracy of map control

by Landsat is discussed.

4.3 Theoretical accuracy

ALS Landsat bulk processed photographic products

are not portrayed with a particular map projection

in mind. Apart from corrections for systematic

influences (e.g. mirror velocity) ALS also corrects

its images for panoramic distortion and earth

curvature (Dovey, 1986). Whatever the along-line

correction procedure may be, the portrayal result

can only be related to an oblique cylindrical

projection with as reference line the ground track

of the satellite. Since the maximum distance away

from this line in an MSS scene is only some ninety

km, the theoretical distortion cannot amount to

very much. As a result, ALS's bulk processed

imagery indeed provides high planimetric accuracy

over large areas as will be shown in Section 4.4.

However, in bulk processing no account is taken

of the direction in which Landsat is pointing

during image acquisition. It is simply assumed

that the satellite is pointing straight down, and

scanning at right angles to its own path. To

obtain higher accuracy than in bulk processed

imagery it is necessary to employ ground control

points with precisely known latitudes and longitudes.

This permits a yaw-pitch-roll correction to be

applied so that Landsat scenes can be produced

with a higher planimetric accuracy (Dovey, 1983).

Unfortunately, however, no precision products can

be expected for the remote areas under discussion

here, since in these areas points with known

latitude/longitude are hard to come by.

Another activity which influences accuracy is

the pinpointing of the photo points in the

Landsat image. The precision with which this can

be done is not limited to pixel size as the points

can be located to within pixel boundaries by

interpolation if the airphoto is used as guidance.

An MSS blow-up to e.g. 1/50 000 scale, of course,

does nothing to increase Landsat's inherent accuracy

but it does increase the accuracy of the cartographer

who has to locate those points within the pixels.

It should be remarked here that Landsat's spatial

resolution, contrary to popular opinion, is not so

much a function of the pixel size, but rather an

expression of the variation in the spatial

information content overlain and 'blurred' by the

pixel grid. Thus defined, Landsat's spatial

resolution is much finer than would be suggested

by its pixel dimensions.

Though it is unfeasible to quantify the method's

accuracy on the basis of the above information, it

has been possible, during the last few years in

Irian Jaya, to assess its degree of magnitude

thanks to 1 m accuracy measurements by the

topographical teams.

4.4 Empirical accuracy

In Irian Jaya Euroconsult was in a position to

test a number of times the accuracy of the method

under true field conditions. For six different

areas ranging in size between 40 000 and 80 000 ha

basemaps depicting the drainage network were

produced using the Landsat map control method. The

team also prepared a planimetric map in UTM projection

for each area, showing traverses and secondary

levelling lines (parallel to each other with 1 km

spacing), along which all encountered river and

stream intersections were indicated. The Landsat

controlled basemap and the UTM planimetric map,

both at the same scale, could now be matched by

fitting river crossing to the drainage lines.

As in this case the drainage pattern is a random

one, it follows that the intersection pattern with

parallel lines, is also random. It is remarkable

that this random field of intersection points fits

with the drainage pattern of the basemap as well

as it does. The few mismatches invariably proved

to be caused by mislocations of small streams on

the air photographs. The method of best possible

fit, used to mate intersections with streams,

showed that the mean deviation which occurred was

around 1.5 mm (when compensated for perpendicular

intersection) in random direction, at a scale of

1/20 000.

As ALS uses a process of portrayal which cannot

be defined by a particular set of mathematical

formulae, but which resembles the method as

mathematically described for an oblique Mercator

projection, and as the topographical team's results

are displayed with the assistance of the UTM

projection, a slight systematic mismatch caused by

this difference of portrayal is to be expected.

However, the absence of visible signs of systematic

variation proves that for our practical purposes

this mismatch is insignificant relative to the

position variation caused by Landsat's geometric

inaccuracies and with the uncertainties in the

positioning of the control points in the Landsat

image.

Two other facts become apparent:

* Without topographical work as a back-up, the

method is adequate for the production of

geographic maps with an accuracy requirement of

30 m, and

* the photo interpreter can always, irrespective

of tilt and scale variations, reconstruct the

location of his field data on the airphoto, thus

optimizing the quality of the interpretation.

5 DISCUSSION

5.1 Comparison

It is possible to express the mean error as a

function of control density in a slotted templet

block. Conversely, knowing the mean deviation (1.5

mm at photo scale) in the airphoto map control

method with Landsat, it is also possible to deduce

the control density that would have been necessary

had the slotted templet method been used. Of

course, this comparison is only valid if photo

tilts and scale variation are within ordinary

limits, which as we know is not the case.

Trorey (1947) has proved that the relationship

is based on the theory of errors, from which it

follows that the mean variation of image point

locations is inversely related to the square root

of the number of control points to which the

slotted templet block is laid. Or

e = kit/c)^

where e is the mean variation in mm, k a constant,

t the number of templets, and c the number of

control points. From empirical data Trorey determined

the value of the constant k as 0.16.

The theoretic control density (t/c) can now be

calculated since e=1.5 and k=0.l6. It would seem

that the 1.5 mm mean variation is comparable with

the theoretical accuracy which would have been

achieved with a control density of about 85 templets

per ground control point had the slotted templet

been used, and had airphotos been used with scale

and tilt within acceptable limits.

To achieve a result comparable to that of the

Landsat airphoto map control method when using a

set of photographs like those made available in

southern Irian Jaya, the slotted templet method