Nautical chart production at the Portuguese Hydrographic Institute (IHPT) is following technical and methodological progress in chart compilation and printing. Traditional large-format lithographic offset production (A0+) techniques have always been used in chart production. However, the reproduction material (repromat) has changed from analogue to digital, i.e. from photographic processes to use of electrostatic plotter. In 2005, as a result of a study carried out at the IHPT evaluating alternative means to lithographic offset, the technical solution of print-on-demand was implemented. This paper discusses the technical issues, selection and experience of print-on-demand at the IHPT.



Technical issues
Print-on-demand technology has since 2003 been available at resolution adequate for cartographic purposes. There are on the market several large-format colour printers which combine different printing techniques (thermal, piezoelectric or electrostatic) and types of inks (soluble, eco-soluble, UV-curable or oil).

The inkjet printer offers a printing process without impact. Text and images are formed by the precise positioning of very small ink droplets injected at high speed by the ejectors from a printer-head. The combination of cyan, magenta, yellow and black ink droplets build very precise colour points that, when viewed as a whole, compose the image. Inkjet printers are emerging as the commonly used equipment for large-format printing, justified by attractive cost, easy handling, a wide-range of papers and inks, and wide variety of resolutions, nowadays appropriate for cartographic purposes.



Print-on-demand
IHPT printer selection involved ex-perimenting with systems including thermal and piezoelectric printing, pigmented soluble and oil inks, and different types of paper, to analyse printing quality from the point of view of water resistance, pencil and eraser use, these being fundamental requirements for chart handling by mariners. Printer evaluation was essentially based on printing quality and speed in paper variety and width. The selected printer uses piezo-electric technol-ogy with a resolution of 720dpi, pigmented oil-based inks, the capability to use several types of paper, and a suitable compromise between water-resistance and stability in pencil and eraser use (Figure 1).



Chart Production
IHPT began effective nautical-chart production with print-on-demand in February 2005 and special attention has been given ever since to product acceptance, production cost and production capacity. Print-on-demand charts have been well accepted by the nautical community. Retailers and mariners, either professional or recreational, understood the added value of nautical charts free from manual corrections to Notice to Mariners at the moment of sale. In general it has proved possible to supply the demanded charts in time. Feedback has been quite positive. So far, in a couple of isolated cases some difficulty has been reported in erasing pencil annotations on the charts. Special attention has been paid to this weakness, and experiments with other types of paper and plastic-coated material are under evaluation.
The natural dynamics of marine navigated re-
gions, from the oceans to coastline, from harbour to berth or anchor areas, and actual changes to port facilities, present permanent challenges to hydrographic services in so far as the degree of update of nautical cartography is concerned. The print-on-demand system presents several advantages, both for user and producer:

• advantages for user: each print-on-demand nautical chart produced by the IHPT has a cartographer, co-ordinator of chart edition, responsible for its maintenance and permanent update. This cartographer is responsible for the edition of the current chart and assures the production-print file is kept updated, so that whenever a user buys a chart it is guaranteed that for the corresponding geographic area there will be no further updated information (Figure 2). The nautical chart is printed with all Notice(s) to Mariners affecting it at the time of printing.


• advantages for IHPT: the decision to produce a new chart depends on evaluation of the degree of change and its impact on safety of navigation, based on the most recently collected hydrographic data or high number of corrections. It may be necessary throughout the lifecycle of every chart edition to introduce other changes without the need to substitute the entire current edition. This is already done in print-on-demand, with no need to introduce the Notice to Mariners in every single chart, in general a time-consuming process.