GIS and Remote Sensing BIO8014 Data acquisition
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
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
Manual creation Tabulated data See practical 1 Excel spreadsheet Converted to shapefile in ArcCatalog
Manual creation Tabulated data See practical 1 Excel spreadsheet Converted to shapefile in ArcCatalog
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
Interactive editing ArcGIS Editor toolbar in ArcGIS (practical 1)
Manual creation Digitisation Free standing digitiser
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
Manual creation Digitisation Hard copy
Manual creation Digitisation OS digital basemap
Manual creation Digitisation Georeferenced
Manual creation Digitisation Digitised route
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
Trimble with ArcPad GPS device
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
GPS satellite network
Satellite trilateration If distance from satellite S is known then somewhere on sphere S
Satellite trilateration S With two satellites somewhere on the intersection of two spheres S
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
Satellite trilateration S S S S A forth satellite will create an intersection at only one point, and provide clock accuracy
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
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
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
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)
Fieldtrip GB
Fieldtrip GB
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
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
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
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. http://magic.defra.gov.uk
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
Remote sensed data Very expensive for new data Selected archive data available through Landmap.ac.uk to academic institutions
Further data suppliers UK soil data http://www.landis.org.uk ESRI data http://www.esri.com/data/find-data.html US government GIS data portal http://geo.data.gov/geoportal/ Fieldtrip GB http://fieldtripgb.blogs.edina.ac.uk/
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