Exam questions Flashcards
What is the difference between Raster and Vector data?
Vector: - uses points and line segments to identify locations - uses X and Y coordinates - list of objects - where is what? - space between objects is "empty" Raster: - uses series of cells to represent locations - uses shaded cells (squares) - the surface is completely used - what is where? - there is no empty "in-between area" so there are "no-data values"
Explain the 2 main types and the 4 further distinctions of raster data and list two examples.
Thematic data (land-use or soil data) Continuous data (phenomenons such as temperature, height, satellite data, aerial data
- Nominal data values: display equality or disparity (land-use, geology, soil types)
- Ordinal data values: display hierarchy (e.g. very high to very low) [water quality classes, soil contamination classes]
- Interval scaled data: numerically indicated; comparable distance between values (temperature, substance concentrations)
- Rationally scaled data: has a natural 0; [0-10, 11-20, etc] (income, distances, precipitation)
What is a geo-object? Explain its characteristics.
Digital representation of a geographical entity or phenomenon. Characteristics are:
- Geometry (point, line, surface area)
- Topology (environmental relationships)
- Topic (factual data / attributes)
- Dynamics (change over time)
What is a “Shapefile” and how is it structured and what is the difference to “Feature Class”? Name other open source alternatives to the shapefile.
A simple, nontopological format for storing the geometric location and attribute information of geographic features.
Consists of a main file (.shp), an index file (.shx), and a dBASE table (.dbf). Can further include:
- Attribute index (.atx)
- index for tables; joins (.sbx and .sbn)
- index for tables; links (.aih and .ain)
- metadata (.shp.xml)
- projection of the data (.prj)
Shapefiles are not in geodatabases, mostly saved in a file folder. For “Feature Class” more tools are available to control and edit geographical objects (e.g. topology exams)
Alternatives to shapefile:
- GML (Geography Markup Language)
- KML
- GeoJSON
- GPX
Explain the different types of geodatabases and explain the “objects” that can be stored in a geodatabase.
Personal geodatabase:
File format: Microsoft Access
Storage Capacity: 2GB
Operating system: Windows
Amount of permitted users: 1 editor; multiple viewers
Distributed GeoDatabase functionality: Checkout/Check-in and replication
File geodatabase:
File format: File folder with binary data
Storage Capacity: 1TB per table
Operating system: any OS
Amount of permitted users: 1 editor; multiple viewers
Distributed GeoDatabase functionality: Checkout/Check-in and replication
Scalable geodatabase:
File format: DBMS
Storage Capacity: depends on server
Operating system: any OS
Amount of permitted users: multiple editors; multiple viewers
Distributed GeoDatabase functionality: All types of replication and versioning
Objects that can be stored are feature classes and geometric networks/network feature classes are saved under a feature dataset. All these include:
- raster datasets
- point features
- line features
- polygon feature classes
- junction feature class
- edge feature class
- geometric network (junction and edge feature class combined)
Describe at least 4 different database field types (attribute types) and give at least 1 example (per field type / attribute type) which geologically relevant information based on a geological question (e.g. solid rock exposure or water sampling) could be stored in these field types / attribute types?
Float: value contains 1 sign bit, 7 exponent bits, and 24 mantissa bits - variable number of decimal points (concentration of contaminant in soil)
Double: value contains 1 sign bit, 7 exponent bits, and 56 mantissa bits - fixed number of decimal points (any value [e.g. contamination, ionic strength] that needs to have the same amount of significant digits)
Short integer: Range of values; -32000 to +32000 (age of a water mass)
Long integer: Range of values; -2 billion to +2 billion (age of a geological formation)
Text: can contain any character (e.g. name of a contaminant)
Date: contains date or date/time
Object ID: a long unique identifier generated in geodatabases
BLOB: complex objects like videos and images
Which properties must be defined for a coordinate system?
- The measurement environment; either
i) geographical (if spherical coordinates are measured from the center of the earth)
ii) planimetric (when the earth coordinates on a two-dimensional planar surface)
- Unit of measure (usually feet or meters in the case of projected coordinate systems or decimal degrees for longitude and latitude).
- The definition of the map projection for projected coordinate systems.
- By other properties of the measurement system;
i) Reference spheroid,
ii) a date and projection parameters such as a or several standard parallels
iii) a central meridian and possible shifts in the X and Y directions.
Explain the difference between geoid, ellipsoid and datum in terms of coordinate systems and give examples.
Geoid: called the Surface of the Gravitational field of
Earth defines and corresponds roughly to that mean sea level.
Ellipsoid: also known as a spheroid. Are defined “Only” by the size or conditions of the semi-axes. Tries to best represent the geoid. Can overshoot 43 to 52 meters difference of the geoid. Clarke 1866/WGS84 1984
Datum: Since an ellipsoid can never perfectly represent the geoid in all areas, a ‘datum’ is used to balance regional deviations such as the Alps for Austria.
The datum determines where the center of an ellipsoid is; where the way is to be “counted” (local center points for more accurate geographical representation). Example is WGS 1984 whose reference point is the earths center whereby the ellipsoid is also the datum.
Explain what a geographic coordinate system is.
- Defined using a three-dimensional spheroidal surface for positions on earth.
- Includes angle unit, a prime meridian, and a date (based on spheroid)
- Can be referenced by its geographic length and width
- Longitude and latitude are angle specifications from the center of the earth to on point on earths surface that is measured
The angles are usually given in degrees - Only the prime meridian and equator are roughly the same circumference and there longitude and latitude can be real units of measure but the further north and south you go, the bigger the error.
- the graticule is formed by a grid of longitude (north-south) lines and latitude (east-west) lines.
- The point 0 for latitude, half between south and north pole, becomes the equator.
- The point 0 for longitude, prime meridian, can vary for countries who are using the GCS and can go through Bern, Bogota, Paris, but more commonly Greenwich, England.
- Origin of the graticule (0,0) is where the equator and prime meridian intersect
Explain WGS 1984 and ETRS 1989 and their differences.
- World Geodetic System (WGS) 1984 used for the entire earth.
- 1984 is the last year that there was a major revision to the system
- Based on global ITRS (International Terrestrial Reference System) = geophysical measurements of the earth
- ITRS and WGS 1984 are tied to the center of gravity of the earth and therefore the measured points change over time due to continental drift
- European Terrestrial Reference System (ETRS) 1989 used for Europe
- Assumes a static Eurasian continental plate
- Therefore is it only used for Europe and independent of time
- 1989 refers to the year when ETRS and ITRS agreed
- Since 1989 due to continental drift there has been a difference between ITRS and ETRS of roughly 50 to 60 cm, which is dynamic and can increase by 2.5cm per year
Explain what a projected coordinate system is.
- flat, two-dimensional surface
- has constant lengths, angles, and areas across the two dimensions
- always based on a geographic coordinate system that is based on a sphere or spheroid
- locations are identified by X and Y coords on a grid with the origin at the center of the grid
- each position has 2 values that reference it to that central location
- one specifies its horizontal position and the other its vertical; origin is x=0, y=0
- x-axis lines and y-axis lines are consistent and equally spaced across the full range of the grid
- distortions can arise because a spheroidal surface has to be placed on a flat surface at a certain size so some aspects have to be represented as smaller than actual
- there are different projected coordinate system types:
- Angled projections
- Equal area projections
- Equidistant projections (distance between 2 points)
- True directional projections
- types of projection:
- Conical
- Cylindrical
- Planar alignment
- Polar alignment
Explain False Easting and False Northing and explain the meaning behind them.
These are measurement offsets to the northings and eastings to avoid using negative numbers that are commonly attained from y and x axis’ (left of the x axis and below the y axis)
They are expressed in coordinate units, not degrees.
Another purpose is to get the same amount of digits for each coordinate
Explain the UTM coordinate system and its specifications.
Universal Transversal Mercator System
- specialized application of the transverse mercator projection (cylinder projection, also called Gaussian-Kruger projection)
- globe divided into 60 north and south zones
- each zone comprised of 6 degree longitude in width
- the zones are numbered 1-60, beginning at 180-degrees longitude and increasing to the east
- false easting of 500,000 meters is applied
- north zone false northing of 0 while south zones have a false northing of 10,000,000 meters
Explain the BMN coordinate system and its specifications.
BundesMeldeNetz is the historically used projected coordinate system for Austria that is the most accurate to this time
- Uses ellipsoid Bessel 1841
- Datum: MGI = Military Geographical Institute of Austria
- 3 zones; meridian strips M28, M31, M34, the numbers refer to the degrees east from the prime meridian Ferro = today’s La Gomera on the Canary Islands
- 1 False Northing for all zones (-5,000,000m), here the values in the Y-direction have 6 digits because everything in Austria is more than 5million meters north of the equator
- 3 False Easting per zone; M28 +150,000m, M31 +300,000m, M34 +450,000m
- there are each different false easting values so no coordinate in Austria occurs multiple times. This would happen if every zone had the same false easting, therefore one knows which zone is specified just by the false easting given
- there is a modified system to the BMN:
- MGI Gauss Krueger 28, 31, and 34 (MGI AUSTRIA)
- has no false easting so there are also negative coordinates and you need the zone for localization
- often used by GPS devices
- Austria Lambert: also uses Bessel 1841
- applies to all Austria (no zones) and has a false easting and northing of +400,000m
Explain the differences in elevations above sea level and give examples.
Different elevations depend on the country using the system. For example:
- Germany: uses Amsterdam Ordnance Datum
- Austria: uses Meters above the Adriatic (Pegel Triest) which is -34cm of Germany
- Belgium: uses gauge in Ostend which is -230cm to Germany
These GPS heights are measured from a global ellipsoid and therefore there can be meters above sea level of 50m in the alps (which makes no sense)
GPS devices have a geoid model stored to calculate differences for more accurate measurements of height above sea level (accuracy of 3-5m)
Barometric correction can be used for more precision
