Data collection and input techniques in GIS. Different methods of data input for gis.

1. DATA COLLECTION AND
INPUT OVERVIEW

2. • The process of getting the data into computer
is called as data encoding in gis.
• The process of data encoding and editing is
often called as data stream.
• The first step in creating a database in gis is to
acquire the data and place them in the system.
• GIS must be able to accept wide range of
datasets.
• Often user generates his own datasets in gis.

3. • There are number of methods for inputting the
data into gis.
• This lecture focusses on different types of gis
data, various methods of data input, different
types of errors in data, editing the data.

4. • Obtaining data is an important part in GIS.
• GIS data is obtained from various sources,
however they are classified into 2 types.
– Primary data sources
– Secondary data sources

5. • Primary data
– Data measured directly by surveys, field data
collection, remote sensing
• Secondary data
– Data obtained from existing maps, tables or other
data sources

6. Primary data
• We cannot usually observe the spatial
distribution of a variable throughout the study
area
• Therefore we need to sample:
– Take measurements of a subset of the features in
the area that best captures the actual spatial
variation

7. • We cannot usually observe the spatial
distribution of a variable throughout the study
area
• Therefore we need to sample:
– Take measurements of a subset of the features in
the area that best captures the actual spatial
variation

8. Sampling of primary data
• The sampling density determines the
resolution of the data
• Samples taken at 1 km intervals will miss
variation smaller than 1 km
• Standard approaches to sampling:
– Random
– Systematic
– Stratified

9. Random samples
• Every location is equally likely to be chosen

10. Systematic samples
• sample points are spaced at regular intervals

11. Stratified samples
• Requires knowledge about distinct, spatially
defined sub-populations
• More sample points are chosen in areas where
higher variability is expected

12. Secondary data(existing datasets)
• More and more ready-made digital GIS data
sets become available
• Government agencies: census geography
• Topographic surveys
• Private companies

13. • Meta-data: “data about the data”
– Procedures used to collect or compile the data
– Data lineage
– Accuracy and measurement standards
– Coding schemes
• Required for both spatial and attribute data

14. • Meta-data often absent
• This leads to
– Misinterpretation
– Misuse
– False perception of accuracy

15. Data sources

16. • The existing datasets in the analogue or digital
form needs to be encoded and should be
compatible with GIS.
• There are different types of data sets available
in the real world and they exist in different
formats.
• Hence, we should use different methods for
encoding and editing the datasets to make
them compatible with GIS for extracting the
information.

18. • Not always data is directly available, in many
situations we have to create the datasets
ourselves.
• Some times data becomes obsolete, there also
we will create newdata for an area.
• The data created will be in many forms like in
the form of drawings, photographs, survey
statistics, survey data etc.

19. DATA INPUT METHODS
• Before discussing data input techniques, its
important to know various types of data.
– Analogue (non-digital) spatial data
– Digital spatial data

20. • Analogue data are normally in paper form and
include paper maps, tables of statistics and
hardcopy aerial photographs.
• All these forms of data need to be converted to
digital form before use in a GIS.

21. • Digital data like remote sensing data are
already in compute-readable formats and are
supplied on diskette, magnetic tape or CD-
ROM or across a computer network.

22. DATA INPUT METHODS
• There are four methods of data input which are
widely used:
– keyboard entry
– manual digitising
– automatic digitisation
– Scanning
• Other than the above mentioned methods of
files are directly transferred into GIS which is
called direct file translation.

23. Keyboard Entry Method
• This method is also called as keycoding.
• It is the entry of data into file for GIS at a
computer terminal.
• This technique is used for attribute data that
are available only on paper.
• This technique can be mixed with digitising
process for the creation of GIS database

24. • The attribute data, 'once in digital format, are
linked to the relevant map features in the
spatial database using identification codes.
• There are unique codes that are allocated to
each point, line and area feature in the dataset.
• The coordinates of spatial entities like point,
line and area features can be encoded by
keyboard entry.
– However, it will not be convenient when the
number of features are more.

25. • This method leads to obtain very high level of
precision data by entering the actual surveying
measurements.
– Used for entering land information during
digitization
– Can be used for drawing maps using survey
measurements

26. Manual Digitising
• Manual digitising is the most common method
of encoding spatial features from paper maps.
• It is a process of converting the spatial features
on a map into a digital format.
• Point, line, and area features that form a map,
are converted into (x, y) coordinates.

27. • A point is represented by a single coordinate, a
line by a string of coordinates, and, when one
or more lines are combined with a label point
inside an outline, then an area (polygon) is
identified.
• Hence, digitizing is called as the process of
capturing a series of points and lines.

28. Procedure- Manual digitizing
• The map is affixed to a digitising table.
• Three or more control points are to be
identified and digitised for each map sheet
– Ex:- intersection of roads, historic monuments etc.
• The points are called reference points or tics or
control points.
• The coordinates of these points are known to
the person who is doing digitizing.

29. • The coordinates of these points are used by the
system to perform necessary mathematical
operations and also to calculate coordinates of
remaining features present in the map.

30. Problems with digitizing the map
• The accuracy of the output of the digitisation
depends upon the experience and skill of the
operator and density of points, lines and
polygons of the map.
• Accuracy also depends upon the selection and
distribution of the control points.

31. • Some of the common problems in digitizing
paper maps are:
– Paper maps are unstable; each time the map is
removed from the digitising table, the reference
points must be re-entered when the map is affixed
to the table again.
– If the map has stretched or shrunk in the interim,
the newly digitised points will be slightly off in
their location

32. – Errors occur on these maps, and these errors are
entered into the GIS data base as well.
– The level of error in the GIS database is directly
related to the error level of the source maps
– Maps are meant to display information, and do not
always accurately record vocational information

33. • While performing digitization there is possibility
of errors
– Positional errors
– Overshoots
– Undershoots
– Spikes
– Etc.
• Hence, proper care should be taken while
digitizing a map so that all required features will
be present and no feature will be missing.

34. Scanning and Automatic Digitising
• Scanning is the most commonly used method of
automatic digitising.
• This method is normally used when raster type of
data is to be produced from analogue maps.
• And the scanned image can be used as the base
for digitizing the features in vector format.
• Different types of scanners are used for scanning
the maps.
– Flatbed scanners
– Rotating drum scanners

35. COGO Method
• COGO full form is – Coordinate Geometry
method
• It is a methodology for capturing and
representing geographic data
• This method uses survey measures for plotting
the data in CAD/GIS systems.
• Survey measurements like bearings and
distance are used in this method for plotting
the features.

36. • Coordinate Geometry (COGO) is a method of
inputting surveying or engineering data into GIS,
CAD or mapping softwares.
• These data may be collected in the field using
conventional surveying techniques and
instrumentation, or may be derived from existing
maps, plats, engineering plans, drawings or
records.
• Data collected in the field consist typically of
coordinates of points, distances and bearings
between points, and point identifiers (with
possibly additional descriptions for these points).

37. • The data are observed and recorded through
standard field surveying procedures, such as a
traverse or a series of layout measurements.
• The data may be non-digital (recorded in
notebooks), or digital (recorded on some sort
of total station or data logging device)
• In the latter case the data will be transferred to
office computers through specialized software.

38. • Existing maps and plans include subdivision
plans, showing legal lot boundaries, and
engineering drawings.
• In both cases features are depicted with some
coordinate values, and distances and bearings
or offset distances are shown between the
features.

39. • Thankyou.