Page | 11
1.2.2 Geodetic Datum One of the most important lessons in geodesy is the next step. How do we tie the ellipsoid to the real world? Now h h modlss up,w h o ch o h lwo ld.H s h d fn onof ‘G od c f nc um’: A Geodetic Datum is an ellipsoid of revolution attached to the Earth at some point. There are two types: • Astro-Geodetic (Regional usage) • Global (Global application) W mo f omsmply n llpsod,‘flo ng’ nsp c o ‘g od c f nc d um’. h wo ldw ys odo this, the historic, astro-geodetic (regional), pre-navigation satellite days method and the global method using satellite orbits.
Figure 6 shows how an astro-geodetic datum is established. Astro-geodetic means the geodetic system is set up by direct observation of the stars using very specialized survey instruments. The surface origin is the place where the survey instrument is set up.
The ellipsoid is attached at the observation point and several ‘alignments’ have to take place mathematically:
The equatorial plane of the ellipsoid has to align with the physical equatorial plane
The Polar Axis of the ellipsoid has to be parallel to the physical polar axis of the Earth
The ellipsoid surface has to be positioned so that it matches as closely as possible to the Geoid over the area of interest
Figure 6: Establishing an Astro-geodetic Datum.
In figure 6, note three significant issues: 1. Astro-geodetic (geoid referenced) latitudes are not quite the same as geodetic (ellipsoid referenced) latitudes. hsdu o h slgh dff nc n h nom l o h sp c suf c s. hs s h mod l‘dso on’du o using an ellipsoid. If the region covered is a continent like the USA or Russia, then as the datum network s sp d c oss h l nd h n l s squ s co c ons c ll d ‘ pl c ’ co c ons h o b m d o minimize the distortion. 2. They take much time to observe under demanding accuracy conditions which can be affected by the weather. 3. They are subject to the observation idiosyncrasies of the observer. Relative accuracy between datums established in the same place by different observers can be several hundred meters. The drawback of the astro-geodetic method is that it is only useful over a specified region, and in general cannot be carried across large expanses of impenetrable terrain or water, since inter-visibility is required. Thus in archipelagos, like Indonesia, this can result in a large number of small regional datums none of which quite match with the others. Political boundaries can also mean a multiplicity of datums even in contiguous land masses; West Africa is a good example, where each country has its own unique astro-geodetic datum. Figure 7 shows a global view of continental gon ld um‘blocks’. Egh d ums oco h wo lddo sno s msodffcul,bu nf c h w ll o 100 unique astro-geodetic datums. Figure 8 shows the proliferation of datums in SE Asia.
Made with FlippingBook flipbook maker