An appeal is made to the public to minimise visiting the LandsD's offices in person unless necessary. The public can purchase most map products from Hong Kong Map Service 2.
Due to the special work arrangements, the processing of various enquiries, complaints and applications may be delayed and it may take a longer time to provide a reply.
The department apologises for any inconvenience caused. What's New. News Event Calendar Recruitment. About Us.
Government Slope Maintenance. Survey and Mapping Survey and Mapping. Spatial Data. Agree Disagree. Your browser does not support the video tag. Download MB. Download Milestones in Survey and Mapping.
Download 8. In determining distances and areas the scale of the map has to be taken into consideration. Directions are based on true north, but if you are using a magnetic compass then it must be remembered that the compass needle points to magnetic north, which is different from true north.
The difference between magnetic north and true north is called magnetic declination. Different types of maps: Being a representation of the real world on a limited size of paper means that a map is restricted as to what can be shown. The cartographer has to select what to show and what to leave off.
The cartographer is guided by what the main purpose of the map is, such as a road map, a topographical map or a thematic map. A road map emphasises the roads and towns but little else, while a topographic map, also called a general map, shows as much of the landscape, elevations, roads, towns etc.
A thematic map is designed to depict a specific theme such as the population of various magisterial districts, the occurrence of crime in different districts, or annual rainfall. What can I find on a map? A topographical map shows natural and human made features on the Earth's surface and added to this are names and boundaries of importance.
The features or objects are represented on the map as symbols in different colours as point symbols, lines and areas. The cartographer uses different colours and symbols for each type of object in a way that will make it easy for the map user to identify. Below are examples of what can be found on a topographical map with the standard symbols: Road:.
In order to determine where we are on the Earth's surface, we first need to know the size and shape of the Earth. To make things easier for us we assume that the Earth approximates a smooth mathematical surface. This surface is called a reference ellipsoid a sphere which is flattened at the poles. To assist surveyors and others in their work, a network of approximately 60 permanently marked and accurately surveyed trigonometrical beacons and town survey marks has been established throughout the country, from which the position and height of any other point can be determined.
This network is termed a passive control network since, besides marking an accurately surveyed point, the beacons do nothing to contribute to refining or monitoring its position.
The all familiar trigonometrical beacons with their white pillars and black vanes which can be seen on mountain tops and on top of high buildings and structures represent this passive network. The increased use of satellite based positioning techniques, mainly GPS, has prompted this Organisation to install a network of active GPS base stations, known as TrigNet, which operate continuously in all weather conditions and at all times of the day. Instead of using the passive network, surveyors can use GPS receivers and data from the active TrigNet network to determine their position on the Earth's surface to within a few centimetres depending on the type of equipment used, the duration of observation, the suitability of the site etc.
Positions derived from these networks are not only to make maps but also for the survey of civil engineering projects, such as the building of roads and water pipelines, land parcels for registration purposes as well as for numerous scientific applications. The scale of a map describes the relationship between a distance or size on the ground. Amap has a scale because it is not possible to show the whole area at its true size. Ascale is normally given as a figure consisting of two parts, such as , the first part is a 1 and the second part 50 This means that 1 unit of measure on the map represents equals 50 units of measure on the ground -1cm on the map represents equals 50 cm on the ground.
Scale can also be given on the map as a scale bar, which helps the user to measure distances from the map. Different maps have different scales. A map with a scale of is said to be a larger scale map than a map with a scale of This is because an object on the ground is shown bigger on a scale map than on a scale map. A million scale map would be an even smaller scale map.
However, the smaller the scale of the map the bigger the area that is covered on the map. The cartographer has the task of bringing the real world to the map user. This is no easy task as the space available on the map is limited and the real world must be represented by symbols points, lines and area fills. The process of making the map involves collecting data and making measurements usually from aerial photographs of objects in the real world: This information is then translated into understandable symbols and names and other relevant information is added which the map user can interpret to acquire knowledge about the real world.
The Earth is round while the map is flat and so the cartographer has to project the round surface of the Earth onto the flat surface of the map.
This process is known as a map projection. There are different properties of preserving true shape, area or distance. For a map to be useful it must be a good representation of the real world. This means that as things change, such as a new road or dam or houses, the map must be revised to show these changes. It is important therefore that maps show the date at which the information is valid. It is also these changes that make it necessary for maps to be updated at intervals.
The updated map is shown as a new edition of the map with a new date. Digital topographic information has been captured from the and map series.
This information has been topologically structured and stored as features. The major features such as all roads, railways, built-up areas, rivers and contours of the entire national series sheets is available in a digital format. This information has been prepared primarily for use in geographic information systems GIS but can also be used in computer-aided drawing CAD systems use in CAD leads to loss of attribute data. The applications to which the digital topographic information can be put are wide, similar to uses of a map, with the difference that users can utilise the power of the computer application software that they are using to do spatial analysis.
This valuable national asset will continually be photogrammetrically improved, and updated to satisfy the nation's need for such information. This data is used by many organisations as well as individuals, both public and private, for planning, engineering, social and scientific purposes.
A Digital Elevation Model DEM is a collection of elevations heights in a digital form, giving the representation of the surface of the Earth. The DEM can be used to determine the height of a point or place, the terrain profile between two points, the visibility from a point. The DEM is determined from photogrammetric aerial photographs measurements and computed in a regular grid of elevation points.
The above DEM information is supplied as a raster matrix of elevations with a limit on the east-west extent. The 25m DEM is available in most metropolitan areas and growth areas. As a result of the high demand for this data, the Chief Directorate has an ongoing programme to increase the 25 m DEM coverage.
Digital maps as raster images are produced as digital copies of the printed maps at scales , and other scales. The raster images are produced as a by-product from the computer-assisted cartographic system. These map images are most useful as 'backdrops' in various applications such as navigation systems, decision-support systems and value-added applications using the map as a reference. Digital orthophoto images are obtained by digitally rectifying aerial photographs.
The digital orthophoto is a photographic image of the terrain, but more importantly it is true to scale and therefore accurate distances and areas can be measured. These images are popular as 'backdrops' similar to the digital map, but have the advantage over the digital map in that it is a real image of the terrain and not one interpreted by the cartographer. To assist users in identifying the products required, index maps indicating the available maps and latest aerial photography are printed annually.
Each map covers a different area of the country and it is important to order the correct map. Each map can be identified by its unique number e. Each aerial photograph covers a different area at a particular date.
0コメント