Techniques for acquisition of geometrical data can be as follows:

a) **Resurvey.** This technique embraces field (GPS, total station
and other traditional), or photogrammetric data acquisition and
total reprocessing of the data resulted.

b) **Numerical revision of map.** This involves the total reprocessing
of the results of the original measurements and of those carried
out since then as an extension, for example calculation of coordinates
of points on the basis of field notes.

c) **Manual digitization of maps and other graphical documents.**
This technique embraces measurements point by point on the table
of digitization or from the display.

d) **Scanning of maps.** This process of work consist of scanning
i.e. automated digitization and the subsequent vectorization.

e) **Mixed techniques.** Combination of the enlisted and eventual
other techniques.

If acquisition of attribute data was carried out on the basis of a valid standard, rules, instruction, the publisher and year of publication are to name.

*Geometrical quality* of the data is characterized by the following items:

- a) accuracy
- b) reliability
- c) numerical precision

a) The required accuracy of the data is given by the expected mean square error. The threefold value of the mean square error is the limit of error.

b) We can get information about the actual accuracy of the data by

- the mean square error computed from the adjustment by the method of the least squares in the case of redundancy of the measurement
- by the difference in the case of checking measurements.

- the mean square error computed from the adjustment by the least squares methods doesn't exceed the expected mean square error,
- the difference computed from the checking measurement doesn't exceed the limit of error.

- a) The required reliability of the data is determined by the expected reliability.
- b) The actual reliability of the data can be computed from the adjustment by the least squares method.
- c) A prerequisite for determination of the actual value of the reliability is to have redundant measurements.
- d) The reliability of an information is acceptable if the actual reliability doesn't exceed the value of the expected reliability.
- e) When computing the arithmetic mean of two values the actual reliability can be computed from a simplified model.

*Quality requirements of geometrical data* of the digital
large scale basic map can be classified on the basis of their
content feature into the following four classes:

*T1 class of tolerance:* cities, prominent resort and industrial areas,

*T2 class of tolerance:* villages,

*T3 class of tolerance:* intensively cultivated rural areas,

*T4 class of tolerance:* extensively cultivated rural areas and those with drawn from cultivation.

The points of the digital large scale basic map can be:

- points of the geodetic control networks
- detail points.

- horizontal geodetic control points: Statutes A.5.
- vertical geodetic control points: Statutes A.4.
- geocode: MÉM Order No. 21/1986. (XII.28.), MÉM Order No. 9001/1987. (MÉM. É. 2.).

a)

b)

c)

d)

The *expected positional mean square error* of the detail
points (in dimension cm) is as follows:

T1 | T2 | T3 | T4 | |

H1 | 3 | 5 | 10 | 30 |

H2 | 10 | 20 | 30 | 50 |

R1 | 10 | 10 | 20 | 25 |

R2 | 15 | 20 | 30 | 50 |

The expected positional mean square error relates to the neighboring geodetic control point, regarded as errorless. If a detail point is simultaneously also a boundary point, then of the potential expected mean square error the stricter one prevails.

The *expected mean square error of the dimensions* should
be computed from the expected positional mean square error of
the individual points by applying the rule of error propagation.
When computing the individual points are to consider independent.
The expected mean square error (in unit of measurement cm) of
the dimensions is as follows:

T1 class of tolerance

T1 | T2 | T3 | T4 | |

H1 | 4 | 11 | 11 | 16 |

H2 | 14 | 14 | 18 | |

R1 | 14 | 18 | ||

R2 | 22 |

T2 class of tolerance

T1 | T2 | T3 | T4 | |

H1 | 7 | 21 | 12 | 21 |

H2 | 29 | 23 | 29 | |

R1 | 14 | 23 | ||

R2 | 29 |

T3 class of tolerance

T1 | T2 | T3 | T4 | |

H1 | 14 | 32 | 43 | 32 |

H2 | 43 | 36 | 43 | |

R1 | 29 | 36 | ||

R2 | 43 |

T4 class of tolerance

T1 | T2 | T3 | T4 | |

H1 | 28 | 54 | 32 | 54 |

H2 | 71 | 56 | 71 | |

R1 | 43 | 56 | ||

R2 | 71 |

Also the value of the *expected reliability* is to prescribe.
The value of the expected reliability is threefold the value of
the expected mean square error.

When the digital large scale basic maps are not produced by resurvey, the accuracy and reliability requirements here specified are only partly to meet. Then the host of data may also permit other allowable accuracy and reliability values on the basis of special initiative. The fact and value of reduced permitted accuracy and reliability prescriptions has to be let known unambiguously to the users of the product.

The *expected mean square error of the height* of the vertical
detail points or boundary and other detail points having height too:

- in case of monumented points 3 cm
- in case of easily identifiable points 6 cm
- in case of difficult identifiable points 10 cm

where is the mean declination angle of the terrain.

The changes occurred at boundary points H1 and H2 have to be registered within one month subsequent the changes in the data subset of the digital map.

The revision of the digital basic maps are *recommended*
in order to ensure the proper data quality, at intervals as follows:

T1 class of tolerance (cities, prominent resort and industrial areas) | 5 years |

T2 class of tolerance (villages) | 10 years |

T3 class of tolerance (intensively cultivated rural areas) | 15 years |

T4 class of tolerance (extensively cultivated rural and from cultivation withdrawn areas) | 20 years |

For the digital terrain model the length of the grid cells is to chose that the error of the terrain model is within the allowable vertical bias. That can be ensured by applying a length of 0.5-3 m for the grid cells in function of the declination angle of the terrain.

Jump to the Homepage of Department of Cartography and Geoinformatics, Eötvös University, Budapest!