Lecture 10

Data Sources & Data Entry

Lecture Outline

I. Building a Spatial Dataset (Data Entry Considerations)A. Data SourcesB. Map ScaleC. Errors and Generalization

II. Digitizing (Data Entry)A. Heads-up (On-screen digitizing)B. Hardcopy map digitizingC. ScanningD. The Digitizing Process

III. Coordinate SurveyingIV. Heads-Up DigitizingV. Geocoding

Building a Spatial Dataset(Data Entry Considerations)

1. Determine what data you need and how you will get it.

2. Identify the scale of your data for mapping.

3. Evaluate errors and generalization with digital or hardcopy data.

4. Determine the extent of your mapping area

1. Determine what data you need and how you will get it...

Common Spatial Data Sources:– Hardcopy Maps (Analog Maps)– Digital Data– Coordinate Surveying– GPS Units

2. Project Scale

Determine scale of data sources.– Consider effect of scale on

data quality.– Large scale maps typically

have less error– Small scale maps typically

have more error

Identify the scale of your data for mapping.

Map Scale Error (Ft)

1:24,000 79

1:50,000 164

1:100,000 328

1:1,000,000 3,281

Surface error caused by a 1 mm (.039 inch) map error.

Coordinate Surveying

Creating spatial layers directly from field surveys.Often used for property lines.

DigitizingThe process by which coordinates from a map, image, or other sources are converted into digital format in a GIS.

Digitizing Methods:Hard CopyHeads UpScanning

Hardcopy Map Digitizing

Human guided coordinate capture from a hardcopy map document.Typically employs a digitizing table or tablet.

Scan DigitizingScan imagery into a raster format and then convert into vector format.Often used in conjunction with on-screen digitizing as much cleanup is required.

Skeletonizing

On-Screen (Heads-up) Digitizing

Data are digitized on screen without a digitizing board.More common method

Steps in On-Screen Digitizing

1. Create a new data layer you will digitize into.

2. Add the data layer and digital spatial source (i.e. an aerial photo) to ArcMap.

3. Trace the location of feature boundaries by specifying the location of nodes and vertices.

4. Correct errors

Data Creation

Tracing in nodes and vertices:

Point Mode Stream Mode

EditingUsed to update and fix errors and inconsistencies in data.Examples: undershoots, overshoots, overlaps, etc.Often, printing out maps helps to identify errors you might otherwise miss.Editing Tools– Select, split, update, merge and delete

features.Advanced Editing Tools: – View Toolbars Advanced Editing

Digitizing Errors

Digitizing Errors:

Field Creation

In ArcCatalog– Access Layer Properties

In Arcmap– Stop editing– Open the attribute table– Go to Options…Add Field

Avoiding Digitizing Errors

Running Topology

Set Snapping Options!

Topology

Relationship between featuresHelps to keep data clean.In ArcGIS…applied to vector data models.

Geocoding (Linear Referencing)

The process of spatially referencing addresses.Only as accurate as your reference data set. – Ex. Streets layer.

Geocoding in ArcGIS

1. Obtain reference data source (Ex. street centerlines)

2. Format/Prep address table3. Create an Address Locator Service4. Geocode Addresses5. Verify/Review geocoding process6. Re-match addresses if needed