Geog 458: Map Sources and Errors

1. Geog 458:Map Sources and Errors January 11, 2006 Data Collection

2. Outlines • Overview of data collection methods • Remote sensing • Digitizing • Geographic data formats

3. 1. How do you collect geographic information? • Information of geographic features can be captured either directly or remotely • Go out and survey; mostly it measures distance or direction from control points; nowadays GPS is increasingly replacing techniques required for ground surveying • Earth information can be collected without physical contact like using camera mounted on aircraft or electronic recording instrument; such a technique is called remote sensing • If some data (directly unusable like it’s in analog form) is already available, data can be converted into digital data through scanning or digitizing  secondary data capture • If some data (directly usable but in different file format) is already available, only file conversion will be necessary  data transfer

4. Overview of data input method Remote Sensing Primary capture Ground survey, GPS Data capture Scanning Secondary capture Data collection Digitizing Data transfer Data capture vs. data transfer? Primary data capture vs. secondary data capture?

5. 2. Remote sensing • Methods of collecting images of the earth remotely • Techniques used to derive information about the physical, chemical, and biological properties of objects without direct physical contact • Information is derived from measurements of the amount of electromagnetic radiation reflected, emitted, or scattered from objects • Sensors are operated through a variety of electromagnetic spectrum (from visible to microwave wavelength) • Sensors can be either passive (sun) or active (synthetic aperture radar: SAR) • Divided into aerial photography and satellite imagery

6. Group discussions • Group A: What do three key aspects of resolution (temporal, spatial, and spectral) refer to? • Group B: What is the difference between air photo and satellite image? • Group C: What is the difference among air photo, orthophoto, and digital orthophoto quadrangle (DOQ)? • Group D: Why is knowing about spectral resolution critical for particular applications?

7. Three key aspects of resolution • Temporal: how often (e.g. every 12 days) or repeat cycle • Spatial: pixel size (e.g. 1 meter by 1 meter) • Spectral: in which electromagnetic spectrum • Single band: sensor is operated in one spectrum • Multispectral: sensor is operated in multiple bands • Hyperspectra: sensor is operated beyond visible bands • Go to EROS data center, and identify three aspects of resolution in different products

8. Air photo vs. satellite image • See Figure 9.2 (p. 203) • Spatial resolution: higher for air photo • Temporal resolution: more variable for air photo • Spectral resolution: more variable for satellite image • Air photo is usually analog; satellite image is intrinsically digital • Air photo is updated in an ad-hoc manner; satellite image is updated on a regular basis (due to earth-orbiting) • Air photo is good for small area (due to high spatial resolution) • Satellite image is good for large area (due to the availability of systematic global coverage)

9. Air photo, orthophoto, DOQs • Air photo is the photo taken from airplane • This does not have a uniform scale, thus you can’t measure distance correctly using this • Orthophoto is planimetrically correct air photo • Rectification to correct for camera tilt • Conversion from central to parallel perspectives to correct for radial displacement • DOQs: orthophoto is scanned, georeferenced using control point, and indexed to quadrangles

10. Advantage of using active sensors • Until very recently, biggest enemy of reading satellite image was cloud cover • Sensors can be reliant on active light source such as radar • Thus information can be collected regardless of weather condition

11. 3. Digitizing • Manual digitizing: digitizing features on hard copy map using digitizing equipments • Heads-up digitizing: digitize vector objects manually straight off a computer screen • Vectorization: the process of converting raster data into vector data; can be either in batch or semi-interactive mode; batch program use some algorithms that extract spaghetti line strings from the original pixel values

12. Measurement error • What is • Overshoot • Undershoot • Dangling segment • Sliver • See Figure 9.9 to answer • How to deal with it? • Data cleaning is necessary (for example, GIS software provides options for snapping, setting tolerance and so on)

13. 4. Geographic data format • Different geographic data format has evolved to meet diverse user requirements • CAD file (e.g. DXF, DGN files) is mainly designed to draw graphics • Raster file format (e.g. jpg, tiff, bil, MrSID) have developed different compression techniques • Well-known vector format include shapefiles, Arc/Info coverage, Arc/Info exchange file (*.e00), MIF (Mapinfo), and VPF file • Conversion between different formats involves information loss

14. Spatial Data Transfer Standard • Aimed at providing robust way of transferring spatial data between dissimilar computer systems with the potential for no information loss (from USGS) • Document released in 1992 (revised and approved by ANSI in 1998) contains standards for self-contained encoding of spatial data • USGS-related data is provided in SDTS format (e.g. DLG, DRG, DEM, TIGER/Line) • GIS vendors provide import software

15. This Friday • No class • Lab handout is posted in the course website • You can do the lab anytime anywhere • Lab should be done individually • I will have an extended office hour 9:30-12:00 in my office this Friday to answer any possible questions • But I would not make it to the class on Friday (2:30 – 3:50pm)