EG1106: GI: a primer

Field & Survey data collection19th November 2004

Topics Choosing a correct surveying

strategy Ground Surveying Techniques Aerial surveying techniques Remote sensing techniques

Practical today

TOPIC 1Choosing a correct surveying strategy

Choosing a correct surveying strategy It is important to ask “What is the

nature of the end product?” also “What format and level of accuracy do we require?”

We may select a surveying strategy that is best suited to these questions

One other important consideration is scale which determines our choice of field equipment

Choosing a correct surveying strategy Sometimes ignorance of existing

resources (such as good aerial photographs) leads to un-necessary fieldwork

Conversely, it would not be appropriate to use remotely sensed images or aerial photos if a high resolution study was required for a local area

TOPIC 2Ground Surveying Techniques

Ground Surveying Techniques Ground surveying is the process by

which terrain data is obtained by direct measurement in the field

Ground surveying produces primary data – whilst aerial and remote sensing makes use of secondary data sources

Ground Surveying Techniques We must work within a framework

– the bounds of which may be measured using tapes

Smaller regions within the framework can be surveyed in greater detail

For efficient resurvey or change detection work, it is important to have a strong control framework

Ground Surveying Techniques

Types of framework

Ground Surveying Techniques

Offset or rectangular fixing method:

Ground Surveying Techniques Independent checks – either by using a

different method or by a different observer ensure no serious errors arise.

The best type of check is one that is independent of the measurement

e.g. if two angles in a triangle are measured, the third may be found by subtraction from 180º. By measuring the third angle an independent check is available.

Ground Surveying Techniques

Planimetric (plane figure) information only

Height and slope information only Combined planimetric and height

information

Field survey work can provide:

Ground Surveying Techniques Triangulation is a method of fixing points

within a coordinate system Where 2 points are known (these could

be landmarks or accurately determined control points) we can assess the position of a third point

All angles in a triangle must add to a total of 180º: know 2 angles and you can calculate the third easily by subtraction

Ground Surveying Techniques There are many other types of ground

survey technique Trilateration, radiation, traversing etc Instruments such as theodolites,

clinometers, abney levels, sighting levels and alidades etc allow precise field measurements to be made

Techniques used for vertical (slope) measurements are more complex and won’t be discussed here.

Ground Surveying Techniques

Measurements of slope angles and vertical change is more complex and requires an array of field equipment

TOPIC 3Aerial Surveying Techniques

Aerial surveying techniques Ground surveys are required for

detailed information and where the bounds of the survey are localised

Some studies do not require such precision or accuracy – and may cover a much greater geographical area

For these studies, money permitting, an aerial survey can be used.

Aerial surveying techniques Photo-interpretation is a qualitative

assessment of features and their significance

Photogrammetry is a more accurate quantitative measurement of features

Aerial surveying techniques

A basic requirement of aerial surveying is some kind of camera – such as a metric camera

Aerial surveying techniques The camera axis must be kept vertical as

much as possible Successive photographs often overlap by

as much as 60% along the direction of flight

Cameras may be loaded with standard emulsion film (usually high definition monochrome) or be sensitive to specific wavelengths of radiation such as infrared

Aerial surveying techniques Aerial photographs initially undergo

rectification to allow for image distortion

Stereoscopes are often used to gain a three dimensional view of an area from sets of stereoscopic photographs

Images can be digitised and georeferenced using a GIS package and data for known control points

Aerial photograph: Liverpool docks area

TOPIC 4Remote Sensing Techniques

Remote sensing techniques There is little difference between aerial and

remotely sensed images (both are a form of remote sensing)

Remote sensing may often (but not always) employ satellites in orbit around the earth

Satellite surveying us useful for very large natural features or surveying large man-made structures such as cities and road networks

Remote sensing techniques

Relatively low cost High refresh frequency Images consistent (same instrument) Different channels (wavelengths) Useful for change detection

The advantages of remote sensing over aerial photography:

Remote sensing techniques

Systems such as Landsat (right) and SPOT provide very good resolution and a wide choice of bands

Remote sensing techniques

Remote sensing can be by aircraft as well - such as radar imaging

TOPIC 5Presenting the Data Cartographically

PRACTICAL TODAY

D1, D2, E1 & E2

Bring coloured pencils and calculator