Mapping the Earth from Space

The first permanent earth observation satellite, today known as Landsat 1, was launched 33 years ago. With a ground resolution of 80 metres, the Landsat Multispectral Scanner images provided an astonishingly comprehensive and panoramic view of many areas of the Earth never before mapped. The ground resolution of the US Landsat family has subsequently been improved, first to 30 metres with the launch of Landsat 3 and then to 15 metres with Landsat 7 ETM. Nearly twenty years ago, in 1986 to be precise, the French/European SPOT (Satellite Pour l'Observation de la Terre) satellite was put into orbit. But the millions of images produced by the Landsat 1-7 and SPOT 1-4 families did not bring us a better-mapped Earth. Today up to 80% of the world's land surface remains unmapped at 1:50,000 scale or better.

Desperately Outdated
In many countries, refusal to embrace opportunities offered by the then new space technology was often the outcome of restrictive federal or state regulation. Indeed, many governments did not understand, or when they did they underestimated, the value of (geo-)information for enhancing the living standards of all their citizens at all levels in society. Today the harvest of such reluctant government policy may be tasted in many countries; large and medium-scale maps have either not been produced or when in better times they were produced - perhaps half a century ago - they have since become desperately outdated and hence of very little practical value. And at the end of the day the truly bitter harvest is that lack of proper geo-information makes any initiative to improve human living conditions cumbersome or impossible.

Millennium Blessings
The new millennium has brought at least three good things for mapping the Earth from space; these blessings carry the names SPOT 5, Ikonos and Quickbird. All three are space-borne sensors that record images of Very High Resolution (VHR): that is, better than 5-metre ground resolution, sometimes also called Ground Sample Distance (GSD). The SPOT 5, Ikonos and Quickbird sensors record panchromatic and multispectral images, ground resolutions in the panchromatic mode being respectively 5m, 1m and 61cm. The resolution of the panchromatic band is typically four times better than that of the multispectral bands; the ground resolution of the Ikonos multispectral images is, for example, 4 metre, whilst it is 2.44 metre for Quickbird multispectral images. Pan-sharpened multispectral images with such a high level of resolution are well suited for mapping purposes and are created by fusing the panchromatic band with three multispectral bands so that a true or false-colour image is obtained with the geometric fidelity of the panchromatic band. With a resolution of one meter and better, buildings, roads, bridges and other detailed infrastructure are well identifiable. What a resolution better than one metre means for the topographic information content of space imagery is convincingly demonstrated in the feature article by Volpe and Rossi (see page 13 of this issue). Using a rigorous model and ground control points, precision is achieved which meets scale 1:5,000, and better, mapping requirements.

High-precision Images
The SPOT 5 satellite was launched on 4th May 2002 and orbits at an altitude of 810km; scene sizes are typically 60km by 60km. When processing the panchromatic and pan-sharpened images in high-precision mode using ground control points, a Root Mean Square (RMS) precision can be achieved of 5 metre. The Ikonos-2 satellite was launched in September 1999 and has been delivering commercial data since early 2000. Processing as outlined above can achieve RMS precision of 1,9 metre. QuickBird was launched on 18th October 2001 and the scene sizes delivered as standard are 16.5km x 16.5km and 16.5km x 165km. The sensor produces the world’s highest resolution satellite imagery commercially available. Processed in high-precision mode, the RMS location precision is one metre. About one billion square kilometres of the Earth’s surface has already been thus recorded.

Continuous Plan
High-resolution satellite images provide an up-to-date and cost-effective means of producing image maps and derived topographic maps for all areas of the world. The ability to extract from 5 metre to 60-centimetre imagery a wide variety of topographic data and to locate features at an accuracy of up to one metre provides an unprecedented opportunity for the cost-effective production of accurate maps of areas ranging from small cities to entire countries. In many parts of the world human activity is so intensive that ‘the present moment’ lasts no longer than the time needed to pile up bricks, anchor concrete and pave a surface with asphalt. Indeed, within a few years of imagery having been acquired it has lost virtually all its commercial value, as rightly remarked by Hagman in our May issue with respect to aerial imagery. This state of affairs is just as valid for space-borne imagery. As a consequence, the information content of airborne and space-borne images, and hence their value as topographic mapping source, is a swiftly diminishing return.

So the purchase of high-resolution satellite imagery should not be a one-off business, but rather part of a continuous plan, a plan that should be embedded in the greater process of maintaining the geo-spatial data infrastructure.