GEOG 481 Final

Geodatabases - What are they?

A “container” used to hold a collection of datasets  

What types of geodatabases can you create in ArcGIS Pro and what are the size limitations of each?  

Enterprise: known as the multiuser geodatabases which is stored in a relational database and requires the use of Arc SDE (Spatial Database Engine) technology and can be unlimited in size and simultaneously edited by a number of users 

Personal: all datasets are stored within Microsoft Access data file which is limited in size to 2 GB and only allows single-user editing

File: geodatabase is stored as folders in a file system and is held as a file that can scale up to 1 TB in size. This option is recommended over personal geodatabases and is only for single use editing. 

What is a feature dataset?

A collection of related feature classes in a geodatabase that share a common coordinate system. Its primary purpose is organizing related feature classes into a common dataset

How is a feature dataset different from a feature class?

A feature class = collection of geographic features with the same geometry type; analogous to a shapefile (most common ones are points, polygons, and lines)

feature dataset = container that holds multiple related feature classes sharing the same spatial reference

GPS – What is it and what is it used for?

GPS = Global Positioning System, used for navigating and is a component of the worldwide global navigation satellite system owned by the US

What are some common sources of error in GPS locational data?

Ionosphere and troposphere errors, blocked signals, and multipath errors; Locations can be inaccurate due to too few satellites, poor satellite geometry and elevation

What role does GPS play in GIS and geospatial technology in general?

GIS relies on GNSS technologies, including GPS, to ensure quality data in mapping environmental data. Global positioning systems also provide X, Y, and Z data for GIS topographical awareness

How does GPS operate?

Through satellite systems and is composed of  constellation of 31 satellites

What is the ideal satellite geometry for conducting accurate field work using GPS?

Triangulation requires at least 4 satellites for precise measurements

ArcGIS Online – What is it?

A software as a service (SAAS) for users to create and publish web maps through their web browser, rather than on a traditional computer, making data accessible to casual internet users who aren’t necessarily trained in GIS

Advantages and disadvantages of ArcGIS Online compared to desktop GIS applications such as ArcGIS Pro

Advantages = easy to use, ability to collaborate and share, automatically updates, integrates easily into easy to use apps (ArcGIS Field Maps, Survey123, and Dashboards)

Disadvantages = limited analysis abilities, dependant on internet to use, limited customization

Mobile GIS apps (e.g., ArcGIS Field Maps, ArcGIS Survey 123) – What are they?

 Part of the ESRI geospatial cloud and integrate seamless with ArcGIS Pro 

What can you do with Mobile GIS maps?

 Allows users or teams of field mappers to use web maps on mobile devices to capture and edit georeferenced data

Any good uses for mobile GIS maps that you can describe in your own area(s) of interest?

Surf spots, coffee shops in specific city, types of animals found in Bolsa Chica, etc

Site Prioritization in Vector GIS – Examples of typical applications of this type of analysis (what would you use it for), typical workflow, etc.

Land parcel and property boundaries, census data, precise positional data, networks, and utility mapping

Define GIS

A system of data, software, and tools used to map and analyze information (spatially referenced information)

Define Raster

Continuous grid cells known as pixels and only one value is designated per cell. Rasters must be numerical and not text so they use numerical coding. It can also be nominal if using numerical data (ex: area codes)

Define Vector

Discrete objects with feature classes that include lines, polygons, and points, and it has a large database

Continuous vs. Discrete

Continuous = measurable, spanning values (fractions/decimals); Discrete = countable, distinct items (whole numbers)

Slope

The incline (steepness) of a surface and calculates the maximum change in elevation over given horizontal distance 

Aspect

The compass direction that a slope is facing

DEM

Digital Elevation Model; any digital representation of the continuous variation of relief over space

Reclassification

A way to convert raster data into more usable information for use in an analysis; assigns a single value to user-defined range of cell values

Raster data

An array of equally spaced cells (pixels) that can be represented by a thematic map or an image; stores the location and characteristics of each cell; groupings of cells represent geographic features & typically a single raster layer that represents a single theme (elevation, land use, soils, etc)

How is raster data different from vector data?

Raster is used by most imaging systems, can easily be compressed, easy to display and analyze, and has many formats; better to use in forestry, remote sensing, terrain analysis, hydrology, and photogrammetry

Vector data has a more precise representation of reality, can represent precise coordinates, assumes a “feature model” of landscape, and can have many attribute fields; better to use for land parcel and property boundaries, census data, precise positional data, networks, and utility mapping

Advantages and Disadvantages (of raster vs. vector)?

Advantages = relatively simple data structures, compatible with remotely sensed or scanned data, can cover a large area with less effort, good for mapping gradations (continuous), and simple data structures make overlay analysis easier

Disadvantages = requires more storage space on CPU and depending on the pixel size, graphical output may be less pleasing, less precise

Applications of raster? 

Terrain analysis (Slope and Aspect), Environmental modeling (deforestation, crop health), Urban planning, Disaster management (flood modeling)

High/Low Resolution?

High  =  more detail and smaller grid size; Low = less detail and larger pixel sizes

Cell Values?

One pixel is all of one number, not a large database like vaster

Thematic Rasters vs. Image?

Thematic raster = discrete, classified date (land use, soil types), where pixel values correspond to specific categories

Image raster = continuous, sensor-collected data (satellite photos), showing specific brightness. All images are rasters, but not all rasters are images

Cell vs. Grid

Cell = single value that can be either integer or float

Grid = number of rows, columns, cell size, XY coords of origin, and Nodata value

Cell size

 It’s specified by the map units (feet or meters) 

Cell Area (aka spatial resolution)

Typically cell size^2; For optimum resolution, the pixel size should be smaller than the smallest feature you want to study; tradeoffs between small size, storage requirements, and processing time

Rows and Columns

row x columns = total number of cells in grid

Spatial Extent (of a grid)

total number of cells in grid x cell area

200 x 200 grid of 10m^2 cells covers 40,000 cells x 10m^2 = 400,000m^2

Calculate total area covered by a given raster (spatial extent)

Use the Recalculate Feature Class Extent tool, check the properties of a layer, or use the Minimum Bounding Geometry tool to create a polygon representing the extent

GRID

Represents a theme or part of a theme (EX: land use type, soil type, vegetation) and can be detailed themes

DEM

Digital elevation model (simple term for digital topographic data in various forms. Implies x, y, and z of the bare earth but void of vegetation and manmade features) 

EX: Ocean Elevation

DRG

Digital raster graphic with no data

EX: scanned image of US Geographical Survey (USGS) topographic map; image of AP Zones and Faults in La Jolla

GeoTIFF

A TIFF image that contains georeferencing information to locate the raster in the appropriate position

EX:Elevation image that contains georeferencing information

JPEG

An image that is remotely sensed, orthophotos, aerial imagery, and must be classified for data to be used in GIS Analyses (not really recommended to use for map images) 

Continuous vs. Discrete Raster Data – What is each?

Continuous raster = Usually represented by grid with a floating point (non-integer) value (EX: elevation, precipitation, temperatures)

Discrete raster = represents distinct, sharp objects that belong to a class (EX: land use, soil types, political boundaries)

How does continuous and discrete raster data generally work (simple) and give examples for each

Discrete values can only take on specific, separated values while continuous data can take on any value within a defined range or interval 

EX: Continuous = Elevation/Temperature; Discrete = Land cover

Slope and how it can be calculated?

Calculated as maximum rate of change in elevation values of each cell in comparison to EACH of its neighboring cells 

Slope Degree vs Slope Percentage?

Slope degree = commonly used in scientific applications; Slope percent = commonly used in transportation studies (EX - Caution: 6% grade ahead)

Aspect and how it is calculated?

Determined as the direction you would be facing if you were looking downhill on the line of the steepest descent (maximum slope) 

Flow Direction, Other Hydrologic Functions?

The cell water will drain into under the influence of gravity

Reclassification - what it does, when to use it?

 A way to convert raster data into more usable information for use in an analysis; assigns a single value to user-defined range of cell values; used when you might need to reclassify a continuous raster surface so that areas with cell values above a certain “cutoff” value are assigned a single value

Digital elevation model – What is it?

Any digital representation of continuous variation of relief over space, implies X, Y, and Z of the bare earth

How does DEM differ from a digital surface model (DSM), digital terrain model (DTM), canopy height model (CHM)?

DSM includes the bare earth like DEM, but it also shows other objects on the ground (buildings/trees). DTM is a more accurate, enhanced version of the bare earth in DEM. CHM is typically used in forestry and urban planning to measure building/tree height. 

What other raster datasets can be derived from the DEM?

Slope, aspect, flow direction, upslope contributing area, grading and earthwork (mass excavation), incoming solar radiation (insolation)

Georeferencing – What is it?

The process of assigning real world coordinates to an unreferenced raster image

When to use georeferencing?

When a non-georeferenced image needs to acquire a known coordinate system and align with the raster data (reference map)

Why use georeferencing?

It allows you to place an image in its correct geographic location by aligning the data properly with other layers in GIS project

General workflow and what is needed to produce an accurate, reliable georeferenced map. 

Identify a series of control points or links with the x, y coordinates

The points are used to link specific locations on the unreferenced raster dataset with the identical location in the spatially referenced dataset

1. Select georeference tool under imagery tab

2. Add reference data (the control)

3. Add unreferenced raster that needs to be georeferenced 

4. Add control points that have known coordinates OR create links that connect known raster positions to known positions in referenced image 

5. Evaluate links for accuracy 

6. Save georeferencing info when satisfied with alignment 

7. Permanently transform original raster dataset and create new raster with transformation applied

Residual Error – What is it?

 The horizontal distance between the location of a control point on the unreferenced image raster and its corresponding control point location on the reference map after a transformation is applied

Root Mean Square Error (RMSE) - What is it?

Statistical measure of the accuracy of a georeferencing transformation that is calculated by the software as links are created

What is its significance of RMSE in the georeferencing workflow?

It indicates how accurately the transformation raster aligns with the real-world coordinates represented by control points; a lower RMSE would mean more accurate georeferencing

Digitizing – What is it?

Process of manually creating spatial data (map features) in a digital format

When to use digitizing?

To sketch a point, line, polygon feature over a displayed image, such as a map, aerial photograph, or satellite image (roads, buildings, parcels, etc.); spatial points now describe real-world locations

Why use digitizing?  General workflow.

Enables users to create new, customized geospatial data from imagery, update existing layers, and perform spatial analysis

1. Create a new empty feature class in your project file geodatabase. Do this in the Catalog pane.

2. Click on the Edit tab to activate the tab to activate the editing ribbon and toolset

3. Click the Create in the Features group to start your editing session.

4. In the Create Features pane on the right side of your workspace, select the right side of your workspace, select the new feature class that you just created, and the new feature class that you just created. Once you do this, a new one is created. Once you do this, a new window will open that shows the window will open that shows the various types of geometry you canvarious types of geometry you can create.

5. Create Features!

6. Snapping (on the status bar and on the ribbon on the Edit tab)

7. Save edits