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Data Capture

• Primary Sources• Secondary Sources

• Vector• Raster

Overview

• Introduction

• Primary data capture

• Secondary data capture

• Data transfer

• Capturing attribute data

• Managing a data capture project

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Data Collection• One of most expensive GIS activities

• Many diverse sources

• Two broad types of collection– Data capture (direct collection)– Data transfer

• Two broad capture methods– Primary (direct measurement)– Secondary (indirect derivation)

Data Collection Techniques 

Raster Vector

Primary

Digital remote sensing images GPS measurements

Digital aerial photographs Survey measurements

SecondaryScanned maps Topographic surveys

DEMs from maps Toponymy data sets from atlases

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Stages in Data Collection Projects

Planning

Preparation

Digitizing / TransferEditing / Improvement

Evaluation

Primary Data Capture

• Capture specifically for GISci use

• Raster – remote sensing– e.g. SPOT and IKONOS satellites and aerial photography

– Passive and active sensors

• Resolution is key consideration– Spatial

– Temporal

– Spectral

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Vector Primary Data Capture

• Surveying– Locations of objects determines by angle and distance

measurements from known locations

– Uses expensive field equipment and crews

– Can also be performed on an airborne platform

– LiDAR, Radar, etc.

– Most accurate method for large scale, small areas

• GPS– Collection of satellites used to fix locations on Earth’s surface

– Differential GPS used to improve accuracy

Total Station

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Secondary Geographic Data Capture

• Data collected for other purposes can be converted for use in GIS

• Raster conversion– Scanning of maps, aerial photographs, documents, etc

– Important scanning parameters are spatial and spectral (bit depth) resolution

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Vector Secondary Data Capture

• Collection of vector objects from maps, photographs, plans, etc.

• Digitizing– Manual (table) – Heads-up and vectorization

• Photogrammetry – the science and technology of making measurements from photographs, etc.

• COGO – Coordinate Geometry

Scanner

The most prevalent wayTo capture non-digital raster data

Scanners are:•Flatbed•Scrolling•Drum

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Batch Vectorization

Vector Over Raster

‘Heads-up’ data capture means the by-handdigitization of objects in vector format from a raster data source

i.e., tracing by hand-

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Digitizer

(A) undershoots and overshoots(B) invalid polygons

(C) sliver polygons

Types of human errors in digitizing:

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Error induced by data cleaning

Mismatches of adjacent spatial data sources that require rubber‐sheeting

Rubber-sheeting – is a procedurefor adjusting the coordinates of allthe data points in a dataset that allow a more accurate match between known locations and a few data points within a dataset

Rubber-sheeting preserves the Interconnectivity between points and objects through stretching, orreorienting their interconnectinglines

Object edges don’t align

Object edges align afterrubber-sheeting, butsometimes itcauses problemselsewhere

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Flowchart of 3D Raster Display