tailieunhanh - KEY CONCEPTS & TECHNIQUES IN GIS Part 3

Thông tin siêu dữ liệu nhận dạng Citation Mô tả Thời gian Nội dung Định nghĩa không gian tham khảo Trạng thái ngang Hệ thống Phối hợp: Bản đồ phẳng chiếu: Lambert giác hình nón song song tiêu chuẩn. | 14 KEY CONCEPTS AND TECHNIQUES IN GIS Metadata Identification Information Citation Description Time Period of Content Status Spatial Reference Horizontal Coordinate System Definition planar Map Projection Lambert conformal conic Standard parallel Standard parallel Longitude of Central Meridian Latitude of Projection Origin False Easting False Northing Abcissa Resolution Ordinate Resolution Horizontal Datum NAD83 Ellipsoid GRS80 Semi-major Axis Flattening Ratio Keywords Access Constraints Reference Information Metadata Date Metadata Contact Metadata Standard Name Metadata Standard Version Figure 6 Subset of a typical metadata tree coordinate systems. The good news is that most GIS these days relieve us from the burden of translating between the hundreds of projections and coordinate systems. The bad news is that we still need to understand how this works to ask the right questions in case the metadata fails to report on these necessities. Contrary to Dutch or Kansas experiences as well as the way we store data in a GIS the Earth is not flat. Given that calculations in spherical geometry are very complicated leading to rounding errors and that we have thousands of calculations performed each time we ask the GIS to do something manufacturers have decided to adopt the simple two-dimensional view of a paper map. Generations of cartographers have developed a myriad of ways to map positions on a sphere to coordinates on flat paper. Even the better of these projections all have some flaws and the main difference between projections is the kind of distortion that they introduce to the data see Figure 7 . It is for example impossible to design a map that measures the distances between all cities correctly. We can have a table that lists all these distances but there is no way to draw them properly on a two-dimensional surface. Many novices to geographic data confuse