Text 2. The Types of data

In geodesy two types of data must be considered.

Datum is the basis for all geodetic survey work. They act as reference points in the same way as starting points do when give someone directions. For instance, when you want to tell someone how to get to your house, you give them a starting point that they know, like a road or building. Geodesists and surveyors use datum as starting or reference points when they create maps, mark off property boundaries, and plan, design and build roads, bridges and other structures.

Another way to think about a datum is as a set of information that acts as a foundation for other data. For example, when a skyscraper is about to be built, the construction team must first build the foundation. Without this element, skyscraper would be unstable and unsafe. This is the same concept as a datum. While a datum is a mathematical and geometric concept, it acts like the concrete foundation of a skyscraper.

In geodesy, two main types of data must be considered. These data are called the horizontal and vertical datum. The horizontal datum is a collection of specific points on the Earth that have been identified according to their precise northerly or southerly location (latitude) and easterly or westerly location (longitude).

To create the horizontal datum surveyors marked each of the positions they had identified, typically with a brass, bronze or aluminum disk or monument.

These markers were placed so that surveyors could see one marked position after another.

To “connect” the horizontal monuments into a unified network or datum, surveyors have used a variety of methods including triangulation.

The Vertical Datum. The vertical datum is a collection of specific points on the Earth with known heights either above or below mean sea level. In areas far away from the shore mean sea level is determined by the shape of the geoid.

Markers are used to identify known positions in the horizontal datum, round brass plates mark positions in the vertical datum. The traditional method for setting these vertical benchmarks is called differential leveling. This method uses a known elevation at one location to determine it at another location.

One of the main uses of the vertical datum is to measure rates of subsidence or land sinking. By means of the vertical datum officials can determine the true elevation and position of the hurricane evacuation routes.

Exercise 9. Look through the words and try to remember their meaning:

data —дані

surveyor —маркшейдер, топограф

datum —дана величина

to create maps —створювати карти

property boundaries —межі власності

horizontal datum —горизонтальна вісь координат

vertical datum —вихідний горизонт

brass —латунь

to triangulate —робити триангуляцію

triangulation —триангуляція, тригометрична зйомка

vertical benchmark —відмітка, оцінка висоти

differential leveling —геометричне нівелювання

land sinking —усадка землі

subsidence —осідання

evacuation routes —шляхи евакуації

Exercise 10. Answer the following questions:

1.What types of data must be considered in geodesy?

2. How do they act?

3. When do the geodesists and surveyors use data?

4. What is the same concept as datum?

5. How can you explain the meaning of the horizontal datum?

6. What did the geodesists and surveyors do to create the horizontal datum?

7. How can you explain the term “the vertical datum”?

8. What is the main use of vertical datum?

Exercise 11. Translate the following word combinations:

geodetic survey work, starting points, a set of information, mathematical and geometric concept, concrete foundation, horizontal and vertical

datum, southerly location, easterly or westerly location, the horizontal monuments, above or below mean sea level, rates of subsidence.

Exercise 12. Complete the following chart:

Verb	Noun	Adjective
to survey
to refer
to create		creative
to mark	marker
to mean
to build
to connect

Exercise 13. Look through the following verb forms and define the tense, write the infinitive of the verb:

must be considered, do, to be built, acts, are called, have been identified, marked, had identified, were placed, have used, is determined, known, uses, can determine.

Exercise 14. Put verbs in the brackets in the correct form:

1. Datum_______ (to be) the basis for all geodetic survey work.

2. Without this element, skyscraper ________(to be) unstable and unsafe.

3. These data _______ (to be) called the horizontal and vertical data.

4. This method ______ (to use) a known elevation at one location to determine it at another location.

5. To create the horizontal datum surveyors marked each of the positions they__________(to identify), with a brass, bronze or aluminum disk or monument.

6. One of the main uses of the vertical datum ________(to be) to measure rates of subsidence or land sinking.

Exercise 15. Read and translate the text.