Considerations about map-updating and images resolution


Basic principles of image analysis for mapping
Most of the work of map production and map updating is based on the recognition of objects or features present on the ground. After visual photo-interpretation of the images, fieldwork (‘completion’) is always necessary to fill the gaps in the interpretation and to collect information that is not available in the images (function of the buildings, village names, etc.).

Field completion is always necessary, but the amount of work needed to achieve it will vary to a very large extent according to the performance of the interpretation of the images: all undetected (or mis-interpreted) features have to be checked on the field.

Choosing one source of image can thus be seen as defining the best balance between the cost of acquiring better images and the reduction of cost if less field work is necessary. We based our analysis of image interpretability on concepts developed by NATO.

The interpretation of an object is organised in 4 hierarchical levels:

1/ Detection is the discovery of an object without recognition.
ex: there is a white linear feature in this corner of the image.

2/ Recognition is the ability to fix the identity of an object within a group type.
ex: this white linear feature is a road.

3/ Identification is the ability to place the identity of an object as a precise type.
ex: this road is a dual carriageway road.

4/ Technical analysis is the ability to describe precisely the attributes of the object.
ex: a low wall separates the two carriageways.

According to NATO, the resolution requested to achieve the interpretation of different objects is as indicated in table 2 (only objects interesting ‘civilian’ mapping are indicated, with the corresponding minimal scale of aerial photography):



This minimum scale is based on images having a good effective resolution, but the potential of interpretation depends also to a large extent on the contrast of the objects on the background.
  • Detection corresponds to the ability to count the number of objects.
  • Recognition is possible when the object is no more a dark spot but clearly a character
  • Identification is possible when one can read the character
  • Technical analysis corresponds to the analysis of the font used (italics, with or without serif, etc.)
As a rule of thumb, and for non-linear objects like tracks or footpaths, one can consider that well-contrasted objects of complex shape can be more or less detected only when their size is larger than 2 pixels, and identification possible for objects larger than 4 pixels. This is only true with a very good MTF (EIFOV=1.2 sampling interval). With an average image quality (EIFOV=1.5 S.I.), only objects larger than 5 pixels can be detected, and objects larger than 6 to 7 pixels can be identified in a reliable way with poor quality imagery.

Processing
To be used for mapping purpose all the satellite images must be geocoded and orthorectified, it means that digital processing put them in cartographic geometry. This is done using digital terrain model (DTM) and ground control points; both of them can be also derived from satellites systems.

The control points are generally provided through a ground survey conducted using satellite Global Positioning (GPS) techniques. In a very near future the use of GPS for precise positioning of the spacecraft on-orbit and stellar Attitude Recording System (ARS) would permit topographic mapping with a ground control point spacing of 500 to 1000 km (rather than 30 to 50km needed with systems like SPOT) so making considerable savings.

The DTM, used for the rectification, can be derived from existing contour lines or from satellite images. Indeed, thanks to their adjustable viewing angles, some optical satellites like SPOT or IRS can acquire stereographic pairs of images from which DTM can be calculated. Radar interferometry is another way to obtain DTM from images like ERS radar satellite for example. Contour lines and spot heights can be derived from those DTM if they are accurate enough.

Modern cartography use, today, two different work processes. The first one, “classical”, is based on photogrammetric tools and uses stereoplotters, fully digital in the best case, but more generally analytical plotters using always films. This solution needs very high educated and specialised operators, and also needs costly investment in hardware and software for the long term.

The second way of producing maps is also derived from the basic principals of photogrammetry, but it uses cheaper hardware and software based on powerful PCs, which have been recently introduced on the market. These new production chains are based on the use of orthoimages for map production and map updating.


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