Administrative

Creating GIS Data from Non-GIS Sources

Common sources

Raw Imagery

CAD Files

Digitizing

Addresses

Different sources, different methods

Raw Imagery

• Georeference (if necessary)
• Classify (by Color/Spectral Characteristics) or
• Digitize (aka Trace)
  


GeoReferencing Imagery



Note: JPG and TIF images can be directly read into ArcGIS. But by default, they won't have an appropriate coordinate system and won't overlay anything else. (JPEG 2000 and GeoTiff are standard formats that are not always supported but can save coordinate system metadata information along with the image.)

So we need some data with a coordinate system we trust. (Warning: Google Earth, etc. can be *very* imprecise internationally - see error in this image below).

So, now we have a valid coordinate system, but our image is clearly pretty far from being correctly registered.

The solution? The "Georeferencing Toolbar" (View->Toolbars->Georeferencing). This tool allows you to identify relationships (by clicking on the screen):

• If you identify one corresponding location in each data layer, the software computes a simple shift.
• If you identify more than one location, the software does a more complex transform (linear, or fancier).

CAD Files

Can simply "open" most common CAD files directly in GIS (DXF, DWG, DGN)

For example, from a real world project, here are CAD data for a regional plan as created by Fonatur, the Mexican national tourism/development agency.



Important limits:

"attributes" don't come along, only layer names *therefore you are well-advised to know the layer naming/numbering convention* (and if none - a big messy problem).

objects must be "exploded" in CAD before export

solids must be converted into boundary representations

Common problems / solutions

Drawn "to scale", but often without explicit projection information and not North aligned

Solution 1: layer properties dialog allows specification of transformations



Solution 2: "world files" (*.wld) are simple text files documenting transforms



Why bother with "world files"?!!! Scalability: one world file can be replicated and applied to many CAD documents drawn against the same base.

Digitizing - Creating new (georeferenced) Geometry

Vector Data Model - Requires boundaries with X,Y coordinates

• Old method - large digitizing table with 'puck' on top of paper map
• New method - 'heads up' digitizing on-screen on top of image
• Illustrate with QGIS
• Chapter 5 of QGIS documentation:
  https://docs.qgis.org/3.16/en/docs/training_manual/create_vector_data/create_new_vector.html
• First, create empty layer with desired coordinate reference system and extents
• Then, add points, lines, or polygons together with attribute table
  

•  We demonstrate use of QGIS to create a new shapefile of polygons via 'heads up' digitizing on top of one of the orthophotos that we accessed last week from the MassGIS WMS server. (You could just as easily digitize new polygons on top of any other shapefiles that we have used.)

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Deeper integration of geoprocessing services within web pages:

• Example of back end support for large, dynamic databases
• Postgres + PostGIS for SQL compliant, multi-user, relational database management with spatial extensions
• Illustrate use of postgres from QGIS - by redoing example from census lecture
• Thematic map of median personal earnings from 2000 census
• Geography: us_ma_e25blkgrpsct_2000 block group boundaries
• Census 2000 data:
• MAGEO table of geographic identifiers and summary levels
• MA0007 table including LOGRECNO and P85 personal earnings data
• Examine tables on PostGIS server (on VM at Medai lab)
• Note X/Y boundary geometry (of polygons) is included in 'geom' column
• Spatial Reference System (SRS) information for EPSG codes is in a separate table
• Relationship between SRS and census block group boundaries is stored in a view
• Join boundaries, mageo, and ma0007 with SQL query in PostGIS for summary level 150

create table c2000inc as
(select g."LOGRECNO",
        e."P085001", e."P085002", e."P085003",
        g."STATE"||g."COUNTY"||g."TRACT"||g."BLKGRP" AS blkkey
   from census2k.mageo g,
        census2k.ma00007 e
  where g."LOGRECNO" = e."LOGRECNO"
    and g."SUMLEV" = '150');

• Pull result into QGIS, join to boundary layer and thematically map

• These tools are useful for 'publishing' project work

• Enable interactive manipulation of maps within browser, PDF document, etc. without 'running' GIS software

Network Modeling and Analysis

    To generate points from Addresses - 'Geocoding's

What is Geocoding

• Geocoding is a process of creating map features from addresses, place names, or similar textual information based on attributes associated with a referenced geographic database, typically a street network that has address ranges associated with each street segment or 'link' running from one intersection to the next.
• Geocoding typically uses Interpolation as a method to find the location information about an address. 
  • (If the address along one side of a block range from 1 to 199, then Street Number = 66 is about one-third of the way along that side of the block.)
• Data required:
  • Reasonably clean, consistent list of legal addresses (i.e. not too many typos, addresses really exist, etc.)
  • Address range attributes on a linear street network
    • Most commonly from Census
    • More current/cleaner data available from private vendors
• Geocoding is one of many geoprocessing services