Maps are an indispensable tool for most people in their daily lives. The Survey and Mapping Office (SMO) of the Lands Department (LandsD) has produced various kinds of maps, ranging from the traditional 2D paper editions of the Hong Kong Guide to the latest developed 3D visualisation map, for public use. Earlier on, I have talked about the 3D digital map. This time, I have invited other colleagues of the LandsD to introduce the new Hong Kong Guide and give us an insight into what a digital orthophoto map is.

Witnessing Hong Kong’s rapid development for nearly half a century

First published in 1976, the Hong Kong Guide has witnessed land development and transition in Hong Kong for nearly half a century. To keep pace with Hong Kong’s rapid development, the Hong Kong Guide has been published once a year since 2000, as opposed to once every two to three years in earlier editions. Over the years, the Hong Kong Guide has not only been enriched in its map contents with an enlarged book size, but has also come with special themes related to geospatial context since 2001 – including the photomap editions published from 2004 to 2007. In addition, the e-HongKongGuide, an electronic guide book first published in 2017, has been made available for free download by members of the public.

The Hong Kong Guide provides information including detailed maps of Hong Kong with street and place names. With the theme of photomap, this year’s Hong Kong Guide is produced with the orthophotos taken between 2020 and 2021. Assistant Cartographer of the LandsD, Mr KWOK Chun-wan, says that every single map page is preceded by its corresponding orthophoto and labelled with major places, streets and trails, thus enabling readers to have a good and quick grasp of Hong Kong’s geographic environment and the relevant spatial information.

What is an orthophoto?

According to Assistant Land Surveyor of the LandsD, Mr LEE Yi-nok, orthophoto is an aerial photograph that has been geometrically corrected through a process called “orthorectification”. This process removes the geometric distortions of photographic images caused by topographic relief and camera tilt by means of digital photogrammetry technology. To take an example, he says that it can be seen in the aerial photographs that the buildings are displaced away from the centre of the photographs with the positions of building roofs and bases deviated. After “orthorectification”, most of the building displacements have been improved.

Our colleagues would compute the accurate location and orientation of each photo at the time it was taken and then perform “orthorectification” by using computers and photogrammetry software. However, due to the high density of high-rise buildings in Hong Kong and the constraints posed by the film-based aerial cameras and limited computational power in the past, it was difficult to rectify all the buildings. As in the Hong Kong Guide 2007, some building displacements could still be seen in the photo map.

Large format digital aerial camera system

In 2016, the SMO started to use the large format digital aerial camera system, which has significantly improved the efficiency of data capture and the quality of aerial photography. Coupled with the advances in photogrammetry technology, the SMO is able to produce a set of orthophotos with no building displacements specially for the latest edition of the Hong Kong Guide. It can be said that the Hong Kong Guide has come a long way since its 2007 edition.

Ensuring that maps are aesthetic and useful

Besides, more than 2 000 orthophotos have been used for the production of photomaps for the latest edition of the Hong Kong Guide. The SMO has taken into account the extent of revision and colour consistency of the orthophotos to ensure that the maps produced are aesthetic and practical. As a last step, our colleagues would have to put grid lines and markers onto the orthophotos for readers to quickly identify the corresponding geographic locations on the maps.

The map products produced by the LandsD, either in the form of paper maps or online digital maps, are closely related to the daily lives of the general public. The 520-page Hong Kong Guide 2022 is now available on sale and its electronic version e-HongKongGuide is available at the website of the LandsD for public browsing and free download. I am aware that the LandsD will continue to introduce latest survey and mapping techniques to consolidate and publish its map products and various types of spatial data for use by various trades and the general public, so that everyone can better appreciate the convenience a smart city can bring to their daily lives.