GIS
distortion of earth's surface
*can distort true area of features *can distort true distance b/w features *can distort direction b/w features *can distort direction between features *can distort shape of features
CAD
*computer aided design (originally computer aided drafting). *used in many applications to create detailed blue prints and engineering drawings.
editing step
*digitizing *data entry *then editing *lastly: building topology, & joining tables
factors in accuracy of spatial data
*distortion due to projection choice *age *natural variation *extent * density of observations * relevance *attribute errors
choice of cell size depends on
*extent of mapped area *scale of map *degree of precision required *amount of computer power available
Geocoding concepts
*geocoding layer, address table and geocoding service. *interpolation of addresses points based on left0right address ranges *values of a data dictionary (alias table) *geocoding to centroid or random pattern with zipcode
density of observations
*scattered observations may miss important info *ex: soil maps are based on one pit for every ten acres or less
Digital Elevation Model
*this data can be used to create a variety of visualizations of the surface or as an input to other computer models *DEM data is available for the U.S from USGS
Steps needed to build a GIS
1. digitizing 2. error detection 3. editing 4. Building topology 5. adding attribute data
Large or small scale
Any map of 1:50,000,000 or larger scale 1:24,000,000 1:5,000,000 is large scale map. A map of 1:100,000, 1:250,000 or smaller scale is a small scale map.
precision
Extent to which a measurement can be consistently reproduced . The state or quality of being exact.
Attribute data
In GIS, graphics are linked to attribute data for each object. Attribute data is stored in tables.
Sampling
Technologies GPS for creating street centerlines, airborne and ground level LIDAR make dense sampling of elevation possible.
dasymetric maps
a class choropleth maps where additional information is used to map the volume of features.
contents of a field
a field contains data about a single category of things for all members of a group.
political boundaries error
a hard set of data to get right due to annexations, enclaves and boundaries being defined by property lines which are not visible on imagery
vector
a quantity with a starting point, a displacement and a direction
contents of a record
a record contains all the information about a single member of a group with respect to each category
GIS
a suite of computer programs designed to store data about geographic features linked to descriptive attribute data
geoid
accounts for the high and low spots and flattened deformed spheroid that is our earth *shape of earth
raster
cell-based, digital imagery * raster is superior for continuous data
Coastlines
classic case where simplification is needed for generalization when scale of maps change
3 main types of projections
conformal: shows directions between features correctly are maintains shapes over small areas equal area: areas of objects are equal from top to bottom equidistant: distances between features are preserved
more on raster
consists of a regular tessellation of space usually raster employs square grid cells like a tiled floor
data capture
converting data into a form where it can be used by a GIS usually from paper maps into digital topological GIS data.
relevance
data should be as relevant as possible. ex: data should be as relevant as possible
Datum
define the size and shape of the earth and the origin and orientation of the coordinate systems used in maps and geospatial data
manual GIS
developed by Ian McHarg starting in 1950's using multiple Mylar overlay maps for urban planning
label or DOT distribution maps
dots are used to show location and sometimes number of some feature. labels and/or annotation can be added and scale reanges and reference scales. dots can be graduated or proportional
contours
elevation data can be represented with contours
digitizer tablet
employed to trace line work. once an object has been entered, associated attribute data is keypunched in
shapefile
file format created for ArcView in 1990's. Less sophisticated geodatabase format of arcGIS.
National Map Accuracy Standards
for large scale (1:24,000 topo maps) the horizontal accuracy standard is no more than 1/30th of an inch off the map = 75 ft on the ground.
ESRI (Environmental Systems Research Institute
founded in 1969 by Harvard Alumni Jack Dangermond to work on image processing for NASA Arc/info developed
street addressing/ geocoding
geocoding layer and service is needed to render a street address into a geographic location via the process of geocoding
object oriented data-base structures
geodatabase structure which is obejct oriented thus there can be one to many and many to many relationships between geographic features and attributes
arc-gis online
has the advantage that you need not download, decompress, and import geospatial data. you can see your existing layers in relation to new data re-projected on the fly
GIS
has two methods raster & vector
hydrographic data errors
has very poor feature name accuracy in the USA
Issues with sampling
if the diversity of an area is large then more intense sampling is needed
Small scale
implies lack of precision but it could be more accurate
Large scale
implies precise information but not accurate
linear geo-referencing
includes street addresses but also includes pipeline data and highway mile markers
data structures
is how data is stored in a GIS
joining related tables
joining a stand alone table to an attribute table requires there to be a primary key in common.
overlay
layer over a layer, they often include flood plains, protection for water wells and protection for historic districts
infrastructure errors
limited infrastructure data in DLG including mines, roads, schools, & many houses, but only if visible on aerial photography
query of a table
logical expression that selects a set of records that match the criteria
attribute data
lots of errors are common, key punching errors
heads up digitizing
maps are scanned and the image displayed on the computer and then a GIS technician follows the outlines on the screen using a cursor to trace objects and keypunches the attribute data
precision of spatial data
maps derived from aerial imagery are only about as accurate as twice the resolution
projection
mathematical method by which features on the spheroid of the earth are represented in two dimensions
spheroids or ellipsoids
mathematical representations of earth need to be used in GIS
discrete objects
might include a well, a business location, or a crime incident or a school
data types
most common types of data are numerical, character, date
arcgis online
most of the data is posted by users with no QA/QC by ESRI. *metadata can be lacking and the data can be in error mislabeled and incomplete
More on Attribute tables
must contain unique feature identifier (FID) that links to feature on map *also has topological information
important datums
north american datum of 1927 north american datum of 1983
scanner
photo optical device that measures reflected laser light and stores the output as a raster format image file
examples of vector data
polylines, contours, roads, streams, polygons, parcels, vegetation, points *discreta data
Map Scale
relationship (or ratio) between distance on a map and the corresponding distance on the ground. For example on a 1:100000 scale map, 1 cm on the map equals 1km on the ground
layers in raster
same coordinate systems, grid size, and orientation so that they can be set down on top of each other
problems with scanning
scanned maps are only raster images. The computer has no idea what the light and dark dots signify
isopleth
shows lines of equal value for some property
soil errors
soil series maps are not available in digital form in most cases and data is fuzzy
Natural variation
soils, geological mapping units, wetlands, boundaries of bodies of water, and vegetation are all fuzzy or change over the year.
continuous data
soils, geology, land use, land cover, land ownership
cadastral data
some states have state wide data but not all
precision vs accuracy
spatial data can be precise but not accurate, however if spatial data is not precise
TIN (triangulated irregular networks)
special case used for simulation of terrain
raster structure
stored in rows and columns data with differing values for a theme are assigned different values in each grid cell. raster data is continuous
cartogram
the area of feature polygons is adjusted by the magnitude of the theme being mapped. often used for numerical data not categorical data
area class maps
the boundaries are determined not by some independent variable like the existence of a nation, county, census tract or but by the data itself. ex: soils maps
Database Tables
the database is made up of rows and columns like a spreadsheet. The rows are called records, the columns are called fields.
distortion
the most important concept related to projection is distortion
topology
the relationship between different spatial objects
cardinality
the relationship between items in two tables *records could be related one to one, one to many, many to one, or many to many. *it is determined by the destination table
the primary key
the typical contents of the first field it is important to link tables to graphics and one table to another.
chloropleth maps
thematic map where value of properties within a boundary are classified and that set of shades is applied within the boundary
complex raster structures
there are hexagonal and triangular grids
SQL
to query data in a relational database a structured question must be asked. *a programming and command language called structured query language has been developed to ask questions of relational databases
TIGER
topologically integrated geographic enconding & referencing system
buffer zones
tracking what is in the adjacent cells is also important a type of spatial analysis
accuracy
truthfulness of information
chloropleth maps
use existing boundaries and use fill colors or patterns to differentiate these and or show values for these areas
stand-alone tables
useful if data is being periodically updated *requires a unique primary key an object ID (OID) *have no tie to map features
boolean operations
value stored in a table that instructs arcgis to join data together according to rules developed by english mathematician George Boole
polygon overlay
values in each overlapping cell in each layer can be combined mathematically to create new layers.
generalization
when data of differing scales or projections is combined it must be transformed. the change from a large scale to a small scale- generalization
extent
you may not have data covering the entire area of interest