A map is a map is a map, except when it's 1995 and the map at your disposal dates back to 1950. Worse yet, a map doesn't exist. As was the case for three Italians who tried to prepare for an archeological expedition to Sudan in 1989. Cartography is not a priority in this mostly desert land so they eventually turned to satellite imagery to see if they could decipher anything from the sky. It worked. Being able to create a topographical map from a black and white image was the key to their discovery of a lost and ancient gold-mining city in Sudan.
     Very few satellite images, however, lead to an "Indiana Jones" type adventure. In fact, most requests for satellite imagery for mapping purposes pale in comparison to the excitement of an archeological journey and could be easily labeled as "just ordinary everyday" cartography. But cartography is anything but mundane. Often quite laborious, it is a science in its own right. And now, thanks to recent technological developments, the job of map-making is done quicker and cheaper than before.
     We're talking about ready-made satellite image maps (SIMs) produced from optical satellite imagery. And we're talking about custom-made SIMs produced according to client specifications. Map making at its finest.
      And with this latest mapping procedure comes a whole new vocabulary as well, probably unlike anything you may hear in a traditional cartographer's workroom today. "Precision corrected," "edge-enhancement," "destriping," "mosiacking," "pre-stretching," "dodging," - these are the buzz words - the functions that allow a satellite image map to be what it is: precise.
      The concept of obtaining standard satellite imagery for mapping purposes is not new. This imagery is used quite frequently in projects and studies as an additional source of information to chart a particular area. What is new is the opportunity for a client to obtain pre-processed digital or photographic image maps of different countries in the world. Maps at a standard scale and projection that reflect the official map sheet system of the country represented.
     In the words of Dr. Mikael Stern, business development manager at Swedish Space Corporation Satellitbild, "SIMs are for users more interested in their application than in image processing. For example, how many people would buy a car if they had to put it together themselves?"
      What Stern is saying is that to process these maps is no easy task, especially when someone needs different data sets merged such as Landsat and SPOT. And he would know since Satellitbild is a forerunner in the SIMs arena.
      With its headquarters in Kiruna, in northern Sweden, and its ground receiving station in Esrange, Satellitbild has been providing remote sensing products since 1985. Custom-made SIMs have been a part of those products since the beginning. But Satellitbild decided to add a second dimension to the ask-and-it-shall-be-yours SIMs production line. With the collaboration of Eurimage, supplier of worldwide multi-mission satellite data, this year marks the start of the "new and improved" image maps: pre-processed SIMs.
      One of the latest creations to join the high-quality ranks of Satellitbild remote sensing products, these SIMs will be an important tool for clients who lack adequate topographical maps and cannot afford to wait a long time for an updated one.
      "Someone who wants SIMs is someone who wants precision," remarked Dr. Andrea Petricono from the marketing department of Eurimage in Rome. "With SIMs, you can have the most up-to-date data available and a map already made for you. You don't have to mosaic it yourself."
      There's that word again. Mosaicking, or piecing satellite imagery together, is standard image mapping procedure when more than one scene is needed to create a map. It's anything but easy, requiring a client to be very familiar with image processing techniques. Since not every client is, SIMs offer a solution as they can be produced relatively quickly and are easy to use.
      SSC Satellitbild uses only optical data from Landsat and SPOT to create their maps. Several data options and combinations are available to the client such as: one SPOT panchromatic (P) band or three SPOT multispectral bands; three Landsat TM bands or seven Landsat TM bands (digital format only); or one SPOT (P) band and three Landsat TM bands merged. The SIMs are provided in both digital CCT and photographic film format with an accompanying map sheet at full resolution and are 50 x 50 cm in size. Coordinate overlay and map legends are added with ancillary map data found on the right indicating the map sheet name, indices, sensors, projection parameters, error estimates, et cetera. SIMs are also produced in all scales with 23 different map projections to choose from.
      Eurimage will apply its marketing skills to help support Satellitbild in the promotion of these products - a joint effort that should increase the availability of SIMs, especially to those who need them most. "Marketing SIMs in Europe is a way for Eurimage to continue moving in the right direction, that of trying to get closer to the end users and understand their needs," commented Fabrizio Lombardi, head of marketing at Eurimage. "We are also able to supply a useful product to some of our distributors who may not be in a position to create it themselves."
      Whether making a ready-made SIM or a custom-made SIM, most of the processing steps are the same. Satellitbild is one of only a handful of companies that can perform them as the process requires what they call "orbital model correction."
      "This means that the geometric correction of the image is based on the real position and 'tilt' of the satellite in orbit," says Stern. "And it is done directly from the raw image and not from the system-corrected one. Doing all the correction in one step and with more precise parameters, results in a much better geometric quality.
      "There are very few companies in the world that routinely use orbital models for geocorrection, and Satellitbild is the only one which is capable of doing this for Landsat, SPOT, AVHRR and JERS-1 data as well," Stern added.
      Once precision corrected, the image is geocoded, or linked to a certain map projection, scale and reference ellipsoid, meaning that every pixel in the image will correspond to a particular latitude/longitude.
      For pre-processed SIMs this point would be the end of the production line. The map projection and scale would be that of the official map sheet system of that country. The coordinate grid would be overlayed coupled with the standard information relative to any map. And that's it.
      But what about the client who needs a map at a different scale or projection than that offered by his/her country? Simple. Satellitbild continues to work in the way most familiar to them - through customer specifications.
      According to a user's requirements, the image will be geocoded to the specified map projection, scale and ellipsoid. A user can choose among three different levels of processing in order to achieve the map desired: Level A - image is corrected without ground control points; Level B - image is corrected using ground control points; and Level C - a Digital Terrain Model (DTM) is applied to correct for local terrain displacement errors. There are also additional radiometric correction techniques that can be applied. According to Stern, a SIM can be achieved in about a week provided the necessary data is already available from Kiruna and no special requirements need to be observed. It all depends on the need. But considering that the traditional way of updating maps usually renders the map out-of-date, as a new series of aerial photographs are taken only as often as deemed necessary, SIMs can be an attractive option.
      Perhaps that's why a visiting environment minister from Estonia a few years ago showed great interest in the SIMs produced at Satellitbild. He commented that topographic maps had been privileged information until recent years and now there is a substantial demand for up-to-date land cover maps. Or perhaps that's why the Federal Survey Department of Nigeria selected SIMs as a means of updating the rural population distribution status in existing topographic maps when preparing for a national census.
      It's true that airborne photography can and will continue to be used as an information source to create topographic maps. But image maps from satellite data have numerous features that render them superior to airborne data. "SIMs are much cheaper, they have a lower resolution, and normally, they have a better geometric quality than airborne data," said Stern. "Furthermore, they are easily available, which is definitely not true for airborne data in many parts of the world."
      According to Stern, a SIM produced from merged Landsat, with its good spectral resolution, and SPOT, with its good geometrical resolution, is the best satellite image product available.
      However, optical satellites are hampered by weather conditions and thus on cloudy days it is difficult to obtain sufficient data. Clients in frequently cloud-covered regions for example who have the task of making topographical maps face quite an assignment. Their only hope is radar.
      The European Space Agency's (ESA) ERS-1 radar satellite and its Synthetic Aperture Radar (SAR) instrument has been chosen by many to map tropical forest regions and other areas where the sun is a rarity. This sensor can penetrate clouds and also acquire data at night. As a result, many agencies are developing technology in order to create radar products such as image maps.
      ISTAR in France is one of those agencies. Working together with two other French companies, they are focusing on areas where no (or only outdated) maps are available and producing optical/radar SIMs.
      Frederic Perlant, head of the radar department at ISTAR, says they have seen good results so far. He gave one example. "We acquired data over French Guyana, a tropical forest. With ERS-1 there was no problem in obtaining imagery. We had full coverage and the images were good. Perhaps we didn't have all the details that we could've had with optical data had there been no clouds, but the 1:100,000 maps we generated with ERS-1 were very impressive."
      Here again it all comes down to what the user needs, but in general, these maps take about two to four weeks to produce after data acquisition and can be made for any area of the world.
      Perlant explained that the techniques for producing these SIMs still need to be refined as radar data is quite diverse from optical and the use of these data is still at an early stage. But ERS-1's successor, ERS-2, which, at press time, is planned for launch in March of this year, will also carry the SAR on-board, making further improvements to the optical/radar SIMs possible.
      As these SIMs can be used for forestry, territorial planning, environmental studies and cadastral projects, Perlant states that these maps are "the products of the future."
      Stern agrees that optical/radar SIMs will probably be an important product in the future and Satellitbild will start supplying these products as well.
      So if it's maps you need, it's maps you'll get whether they're optical or optical and radar combined. The applications are numerous - urban planning, forest inventories, land cover mapping, natural resource inventories, land use management, or maybe even another lost city to find.

About the Author:
Mary Jo Wagner is a writer/editor at Eurimage in Rome, Italy. She can be reached at +39 6 40694266.

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