GIS and Remote Sensing BIO8014. Data acquisition

Transcription

1 GIS and Remote Sensing BIO8014 Data acquisition

2 Introduction Data can be manually created Data can be obtained from a wide range of providers both free and at cost Acquisition is key and must be accounted for in every project plan both time and cost

3 Manual creation Tabulated data Point records Must contain an X and Y data field; longitude and latitude Some cases a Z value for an amount e.g. height of building or height of wind turbine Must know the applicable coordinate system to which the X Y data refers

4 Manual creation Tabulated data See practical 1 Excel spreadsheet Converted to shapefile in ArcCatalog

5 Manual creation Tabulated data See practical 1 Excel spreadsheet Converted to shapefile in ArcCatalog

6 Manual creation Digitisation The conversion of data to a digital format Slow and dull But essential GIS skill Formerly used to digitised entire maps, rare now as most map data is primarily in digital format See practical 1; refer to the creation of your walk to the library: onscreen digitising

7 Interactive editing ArcGIS Editor toolbar in ArcGIS (practical 1)

8 Manual creation Digitisation Free standing digitiser

9 Manual creation Digitisation How to digitise data of hard copy maps Need to convert paper map to a digital image Scan Photo Georeference; the process of spatially reference an object Vectorise the feature(s) of interest

10 Manual creation Digitisation Hard copy

11 Manual creation Digitisation OS digital basemap

12 Manual creation Digitisation Georeferenced

13 Manual creation Digitisation Digitised route

14 GPS data Original receivers bulky (rucksack carrier) and very expensive Now Trimble, Garmin etc. produce small, hand-held receivers found in; Navigation devices Watches Smartphones More complex ones have data input facilities ArcPad for interactive entry of GPS data in the field

15 Trimble with ArcPad GPS device

16 GPS satellite network Created by the US Department of Defence Originally run using 24 satellites Initially reserved for military use Until 2000 only a degraded signal was available for civilian use (low accuracy ~ 100m) EU Galileo system Russia GLONASS China Beidou navigation system

17 GPS satellite network

18 Satellite trilateration If distance from satellite S is known then somewhere on sphere S

19 Satellite trilateration S With two satellites somewhere on the intersection of two spheres S

20 Satellite trilateration A 3 rd measurement narrows down our position to two points One point can be ignored (it will be in space!). As long as we know the distance from 3 satellites, we can find our position S S S

21 Satellite trilateration S S S S A forth satellite will create an intersection at only one point, and provide clock accuracy

22 Satellite trilateration How do we know distance from satellite? Need to know exact time A pseudorandom signal sent from satellite to receiver Receiver also generates pseudorandom signal Synchronise pseudorandom signals from satellite and receiver and measure the time difference between same part of code Multiply time difference by speed of light to find distance to each satellite

23 Aspatial/Attribute data Usually best entered as a separate process from digitising Only simple ID nos. entered at digitising Raster maps may require different categories to be selected by operator, and identified Large amounts of aspatial data can sometimes be entered in a spreadsheet, before linking with spatial

24 Smartphones General ecological \ environmental survey vs civil engineering? GPS positioning in modern iphone (Apple) and Android phones Requires good mobile network coverage. 3G nationwide, 4G improving Can download maps in advance for use in field if reception is poor

25 Fieldtrip GB Produced by Edina (Edinburgh University) for environmental scientists Simple web interface to customise data collection application on smartphone Displays background map Data synchronisation via Dropbox account on smartphone and PC Stores grid coordinates and attribute data (including photos \ audio)

26 Fieldtrip GB

27 Fieldtrip GB

28 General data errors in GIS In brief, spatial data might be incomplete or double in wrong place at wrong scale distorted linked to wrong aspatial data aspatial data wrong or incomplete Remember errors can be propagated through the GIS database if not corrected early

29 Using extant datasets Usually good investment of time and money Many countries (especially USA) put spatial data into the public domain (free) UK academia has reduced cost or zero costs for some GIS data Cost of labour to create one s own usually means commercial datasets less expensive in the long run Some datasets can be downloaded from web, others CD-ROM delivery

30 EDINA UK national academic data centre based at the University of Edinburgh Digimap; Ordnance Survey, geology, historical maps & marine maps UKBORDERS; boundary data for the UK agcensus; agricultural census data ShareGeo Open; spatial data repository to promote data sharing

31 MAGIC Central web-portal for spatial environmental data produce by governmental organisations Providers include; Defra, Environmental Agency, Forestry commission and many others Data includes; SSSI, Ancient woodlands, Local Nature Reserves, etc.

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33 Commercial suppliers High costs, either added to a given project or supplied by client emapsite is one such supplier OS data Satellite imagery Height data

34 Remote sensed data Very expensive for new data Selected archive data available through Landmap.ac.uk to academic institutions

35 Further data suppliers UK soil data ESRI data US government GIS data portal Fieldtrip GB

36 Summary Data is key and will drive project Creation can be timely Acquisition can be costly You must be able to make an assessment of data quality Spatial accuracy of data for environmental scientists vs civil engineers may allow simpler data collection Websites change rapidly