Digitizing GIS Feature Information

Transcription

1 Digitizing GIS Feature Information

2 Digitizing The process of tracing an analog or digital image to create features (points, lines, or polygons) in a GIS data base

3 Analog images are hard copy images such as paper maps or aerial photos

4 With analog images, features are digitized using a digitizer table connected to a computer running GIS software

5 Analog to Digital Scanning Digital images are images that have been electronically scanned so they can be displayed on a computer screen. BUT, it involves more that just making a digital image. Why? What is missing? Rectifying an image: 1) Tying each pixel to the Earth with a unique coordinate Universal Transverse Mercator (UTM) Albers Equal Area Lambert Conformal Conic State Plane Geographic (Lat/Lon) unequal area Round Earth to Flat Map 2) Calculating what the pixel size will be

6 Analog to Digital Scanning Rectifying a topoquad to Earth coordinates is easier than a scanned photo. WHY? Imbedded reference systems Lat/Lon UTM State Plane

7 Using Arc s Help Function

8 Rectifying a Scanned Image

9 1 st order transform 3 pts best to use no less than 4 2 nd order transform 6 pts best to use no less than 9 3 rd order transform 10 pts I use no less than 25

10 If done rectification is done well The rectified images should overlay precisely with other georeferenced data in the same coordinate system. Both UTM Zone 15N NAD83

11 With rectified digital images, features (points, lines, or polys) can then be digitized directly from the computer screen into a GIS.

12 Steps in Digitizing a New Shapefile Use ArcCatalog to create a new shapefile Add digital image to ArcMap Add new shapefile to ArcMap Set necessary snapping environment Begin editing and digitize features

13 Steps in Digitizing New Information into an Existing Shapefile Add digital image to ArcMap Add existing shapefile to ArcMap Set necessary snapping environment Begin editing and digitize features

14 Geographic Information System Attribute Tables

15 Graphic Images Points, lines and polygons provide useful information Simply visualizing spatial relationships can be helpful But many non-gis graphics programs can do that.

16 Data files Data base files can provide many different kinds of information Data can be sorted, ordered, added to, and manipulated But many non-gis data base programs can do that

17 GIS Provides Both Attributes and graphics are dynamically linked Select one and you get the other Changing one causes changes to the other

18 Each graphic feature has a line in the associated attribute table FID Shape ID 0 Polygon 0 1 Polygon 0 2 Polygon 0 3 Polygon 0

19 Attribute Table Creation Basic table is created automatically by the GIS as features are created Each row in basic table contains FID---Feature ID Shape---Point, Polyline, Polygon ID initially 0 but can be reset by user

20 Can add new fields as needed Field is a column in an attribute table that contains information for each feature Field types can be: Integers (short or long) Decimal numbers (floats or doubles) Text Date Binary Large Objects (BLOB) Global ID s (GUID) FID Shape ID 0 Polygon 0 1 Polygon 0 2 Polygon 0 3 Polygon 0

21 Obtaining values for new fields Some field values can be calculated from the feature layer in GIS Line lengths Polygon perimeter lengths Polygon areas

22 Obtaining values for new fields Some values can be calculated from other fields already in the attribute table ie. GIS can calculate the area of a polygon in square meters. You can then use the attribute table to calculate area in acres by applying the appropriate conversion to the square meters field.

23 Obtaining values for new fields Field values can be typed in manually For example Ground cover types such as corn or beans Names of towns Census information for towns Land owner names and addresses Rainfall amounts Size of tile line outlets etc

24 Obtaining values for new fields Field values can be joined to other existing data base files If you have a unique value in your attribute table for each polygon, point, or line and a database table with information about each point, line, or polygon that has the same identifier the database table can be temporarily or permanently joined to the GIS graphic file as new attributes.