Lecture 5 GIS Data Capture & Editing Tomislav Sapic GIS Technologist Faculty of Natural Resources Management Lakehead University
GIS Data Input Surveying/GPS Data capture Facilitate data capture Final GIS data Aerial/Satellite Images Hardcopy Maps Georefencing; if needed, orthorectifying. Already Created GIS Data Digitizing ( heads up, table), image classifying. Create New or Modify Existing GIS Data GIS User
GIS Data Input: Coordinate Systems and Datums A satellite/aerial image or aerial photo dictate the project. coord. system by the size and geographic position of its extent Surveying and GPS use a chosen map projection/coordinate system and datum. A map should, presumably, have the project coord. System and datum defined. Already existing GIS data was created through surveying/gps or from satellite/aerial photos or maps and should have a defined datum and coordinate system.
Already Created GIS Data GIS data are being created daily by different levels of government, organizations, companies, individuals. The quality of created GIS data (accuracy, precision, extent of metadata) depends on how broad the user base is and how significant and critical is the use of data. In Canada, provincial and federal governments maintain databases of so called basic layers, such as roads, railroads, towns, land ownership, lakes, rivers, and then these basic datasets are used by governmental agencies, organizations, companies to build and maintain their own databases that reflect their own field of interest (e.g., managing forest management units). In addition to databases with basic datasets, more specialized GIS data (fire, soils, climate, history, forests, plant and animal ranges) are today available through a myriad of GIS data portals Any GIS database presumes constant maintenance and update constant editing of data.
GIS Basic Datasets Databases In Ontario, today s GIS database was preceded by the basic geographic description of the entire province stored on paper, 1:20,000/1:10,000, topographic Ontario Base Maps. OBM formed the basis of the contemporary GIS database. Today, OBM data are part of Land Information Ontario (LIO) https://www.ontario.ca/page/land-information-ontario. There is also a federal GIS database of basic features, called CanVec (CanVec+) http://geogratis.gc.ca/site/eng/whats-new/intro-canvec that is based on the 1: 50,000 National Topographic Database tiles and additionally captured data. The CanVec database includes GIS features captured by the federal government but also features carried over from the provincial databases. Governmental GIS databases are moving away from a tiled structure and are becoming seamless.
Ontario Base Map (OBM) GIS Database Entire province is divided into square tiles, 5x5 km in the south and 10x10km in the north. Datasets are created based on a scale 1:10,000 in the south and 1:20,000 in the north. Each tile contains a multitude of datasets, such as roads, rivers, lakes, dams, towers, parks, contours, DTM, and many more. Each tile is named by adding scale + UTM zone + UTM easting of the south-west corner + UTM northing of the south-west corner. Usually the name is shortened by leaving out the scale and sometimes the UTM zone, and by truncating the UTM coordinates. Often OBM layers are referred to as basic layers. UTM Zone 15 UTM Zone 16 163205320 Zone 16 SW corner x = 320000 SW corner y = 5320000 2016 3200 5320 is another format, indicating northern Ontario, 1:20000 scale.
OBM tile files are often provided as.e00 files. e00 files are transfer packets of another ESRI vector file type, coverage, and need to be imported in ArcGIS by following a specific procedure: http://desktop.arcgis.com/en/arcmap/latest/ manage-data/coverages/importing-anarcinfo-interchange-file.htm
NTDB > CanVec > CanVec+ NTDB > CanVec CanVec+ http://geogratis.gc.ca/site/eng/extraction http://www.nrcan.gc.ca/earth-sciences/geography/topographicinformation/maps/9767 http://geogratis.gc.ca/api/en/nrcan-rncan/ess-sst/23387971- b6d3-4ded-a40b-c8e832b4ea08.html
GIS Data Editing Editing is a process of adding, deleting, and modifying spatial and attribute components of GIS data. Editing in GIS is almost exclusively done on vector GIS datasets. Raster datasets are very rarely edited. Editing is carried out to capture data, correct errors, or to update GIS datasets. GIS data changes over time and unless a GIS dataset is a historical record, some of its parts eventually need to be changed or new added through editing. As opposed to, e.g., applying symbology and displaying only certain parts of layers, changes introduced to datasets through editing, change the datasets themselves.
Vector Spatial Editing: Tolerance If one coordinate is within the tolerance distance of another one, both are given the same coordinate value. Tolerance can be set for datasets (e.g., feature classes in geodatabases) or for editing processes. The default tolerance in ArcGIS is set to 0.001 meters or its equivalent in map units.
Vector Spatial Editing Editing of features is associated with the vector data model; direct spatial editing of individual raster cells is not possible. In ArcMap an editing session is conducted within one workspace, i.e., folder or a geodatabase. A dataset (layer), meaning the workspace in which the dataset resides, or the workspace itself is chosen for the editing.
Vector Spatial Editing: Selections An editing session often occurs with multiple layers whose features overlap and individual layers containing many features. Some editing tools perform tasks on all selected features or require only one feature to be selected. Errors small and of great significance can be easily made and not noticed. Selectable layers and selected features should be controlled to avoid errors.
Vector Spatial Editing Editing is a separate process that is started and ended within an ArcMap project Edits have to be saved within the editing mode (e.g., the Editor toolbar in ArcMap) saving the ArcMap project is unrelated to editing and it does not save editing changes! In ArcMap, editing is started from the Editor toolbar. Edits need to be saved frequently and editing stopped at the end.
Vector Spatial Editing Spatial editing of vector GIS features means adding, deleting, and modifying vector features. Since the spatial basis of all GIS vector features are x,y points (vertices), spatial editing means making changes to the points or vertices. These changes are then observable on the screen as changes in location or shape. A modified group vertices now represents the polygon A new polygon is created.
Vector Spatial Editing When new spatial features are created, new records (rows), representing newly created spatial features, are added to the table. The last table record before the feature addition. A new record is created after the feature addition.
Vector Spatial Editing Editing Tool should be used with extra caution. For example, a significant (catastrophic) error can be made by working on a small area in a layer containing features across a large geographic extent and (by mistake?) selecting all features in the layer (all lakes and wetlands in Ontario?!) and using Editing Tool to move what looks like only one lake but are actually hundreds or thousands of lakes and wetlands.
Vector Spatial Editing: Snapping Snapping forces the cursor to connect to existing features, their vertices (points) and edges within the snapping tolerance. This way, misconnections (undershoots, overshoots) are avoided. This is used in adding new features or modifying existing ones. Snapping occurs within the tolerance set at the time. Snapping point end vertex edge Snapping toolbar The cursor has been connected to the edge of the polygon.
Attributes Editing in Vector Files Editing of attributes in raster cells is almost never done. Similar to editing features, attributes editing in vector files involves adding, deleting, and modifying attributes in GIS datasets.
New Field Values, Area and Length Fields Whereas in shapefiles new fields and records by default get assigned a (empty space) or a 0, depending on whether the target is a text or numeric attribute, in geodatabase feature classes a Null (values absent) is assigned. Shape area and shape length (line segment length, polygon perimetre length) fields are automatically added to gdb feature classes, according to the feature class features geometric shapes (area and length fields to polygon feature classes, length to line, and neither to point feature classes), and are live-linked to the features. However a great cation should be exercised in recognizing remnant fields from shapefiles, if the feature class was created by importing a shapefile to the gdb. Fields carried over from the imported shapefile Fields newly created in the feature class
Vector Spatial Editing: Topology Topology is the arrangement that defines how point, line, and polygon features share coincident geometry. Features within and between GIS datasets share geometry based on the rules defined or implied in the design of the dataset. For example, stand polygons are not supposed to overlap, lake polygons are not supposed to overlap land or wetland polygons, fish spawning points should be within lake polygons, and so on. The fact that geodatabases are object-relational databases means that spatial objects such as topology can be created in, e.g., a feature dataset, and linked to feature classes. Errors found through validating features in datasets (do the features comply with the rules or not?) are then fixed through editing. Compliance with the rules is done based on the set tolerance. Examples of topological rules: Must Not Have Gaps Must Not Overlap Must Not Overlap With http://help.arcgis.co m/en/arcgisdesktop/ 10.0/help/index.htm l#//001t000000sp00 0000.htm
References: ESRI. 2012. ArcGIS 10. FGDC. 1998. Geospatial Positioning Accuracy Standards, Part 3: National Standard for Spatial Data Accuracy Foote, K.E. and D.J. Huebner. 2000. The Geographer s Craft Project. The University of Colorado. Geomatics Canada. 1996. Standards and Specifications of the National Topographic Data Base. http://ftp2.cits.rncan.gc.ca/pub/bndt/doc/stdntdb3_en.pdf Heywood, I., S. Cornelius, and S. Carver. 2006. Introduction to Geographical Information Systems. Pearson Prentice Hall, New York. Minnesota Planning. 1999. Positional Accuracy Handbook. http://www.mnplan.state.mn.us/pdf/1999/lmic/nssda_o.pdf US Forest Service. 2003. Draft GPS Data Accuracy Standard. http://www.fs.fed.us/database/gps/gps_standards/gps_data_standard.pdf