Intro to GIS lecture 7 Data for GIS

In Situ Data collection

1. Digitation of Historical Spatial information. 2. Global Positional System (GPS) 3. Traditional Land Surveying 4. Surveying Using a Total Station 5. In Situ Data sampling or the taking of a census

Advantages of heads up digitizing

1. Many data sources are digital ( may be magnified on screen to any desired scale) 2. Converting image to a paper or other hardcopy form would likely introduce error 3. On-screen digitizing is often more accurate than canal digitizing because manual map digitizing is often limited by the point ability of the operator 4. On-screen digitizing may be used to limit operator-induced positional error when digitizing 5. On-screen digitizing also removes or reduces the need for digitizing tables or map scanners.

Steps for heads down digitizing

1. Tape map to the digitizer 2. Register control points on the map 3. Estimate two conversion equations (one for vertical and one for horizontal coordinates 4. Digitize vectors (points, lines or polygons)

Scanning and vectorization

A document is scanned using a digital scanner. The document is fed through an opening in the front. Light is emitted onto the document, and the reflected light is read in by a sensor. The sensor then records the reflected brightness to create an image. Specialized software is used to convert the scanned image to lines. This proves is called raster to vector conversion.

Plane - table survey

A special telescope alidade is placed on a leveled plane-table. A line is drawn from the center of the table in the direction of feature of interest, outcrop, tree etc * ideally placed over monument that had X,Y coordinates. Surveyors repeated procedure at second stations and measured the distance between stations. A second member of the surveying crew wold go to a station 3 and hold a leveling rod (stadia), the surveyor at home station could view the stadia through alidade and determine the change in elevation.

Total Station

A theodolite with an electronic distance measurement device, that can be used for leveling when set at the horizontal plane Needs local or federal control points. A SmartStation uses a GPS-Enabled that uses dual frequency GNSS receivers and real-time kinematics (RTK) technology A SmartPole is a GPS enabled stadia rod. Both SmartStation an SmartPole can be used independently to acquire accurate x,y,z measurements.

Scanning the map

A user can scan the map at a high resolution

Heads up digitization of a map in lab

ArcCatalog>CreateNewFeature>AddSpatialReferefereinceInformation>AddnewFields

Heads up Digitizing

Combination of scanning and manual digitizing Scanning the map, Registering the map, Digitizing the map

Digitizing new features

CreateBasemap>AddFeatureinArcMap>StartEditing>DigitizeFeature>StopEditingSave

Snapping (lines)

It is virtually impossible to play a name at the exact intersection point with a line. Line snapping takes nodes and snaps them to a nearby line if the nearby line is within a specified tolerance.

Heads up digitizing

Mouse on a screen, Digitizes paper maps, aerial photos, or other images.

Tablet digitizing - Feature Collection

Point mode : Allows the operator to place discreet points to represent geographic objects. Stream Mode: Automatically places points at specified time intervals and the user traces the digitizing puck along geographic features

Tablet digitizing

Requires requires a person to enter coordinate information through the use of a digitizing table and digitizing puck.

Snapping (nodes)

Similar to line snapping, however node snapping snaps nodes to nearby nodes within a tolerance.

Traditional land surveying

Simple plane-table instruments

Methods for Digitizing

Tablet digitizing, Heads-up Digitizing, Scanning and Vectorization.

Digitizing

The process where features on a paper map or image are converted into digital format for use by a GIS is called Digitizing. ** Converts feutres on the map into three basic data types: Points - Zero dimensional objects Lines - One dimensional objects Polygons - Two dimensional objects

Digitizing the map

The user can zoom to specific areas on screen and trace points, lines or polygons on the map because the maps are already in the correct geographic coordinate system anything digitized on top of the map will also be in the correct coordinate system.

Heads down digitizing

Used to digitize hard copy maps into GIS Transform wire intersection into coordinates of the tablet's coordinate system.

Registering the map

Using the same transformation methods previously discussed, the user can enter control points on screen and transform the scanned image to real world coordinates

Most popular spatial reference information

WGS 1984, whenever creating new feature class.

Tablet digitizing - Digitizing errors

Weeding, Snapping (lines), Snapping (nodes).

Weeding

When vertices are entered too close together the GIS will remove some of them in order to reduce the file size