GEOG 481 Final Exam Flashcards

Topology

A mathematic study of “knowing what’s next to what”

“Raster is faster, vector is corrector”

With raster, you’re able to perform analysis and calculations faster and is more performance-efficient, while vector is known to be more precise. Choosing between raster and vector depends on the type of analysis you’re doing.

Continuous data

Data exclusive to raster GIS that generally have high spatial autocorrelation (gradually changing values).

• Ex: Elevation, slope, flow accumulation

Discrete data

Data representing distinct spatial objects (belonging to a class)

• Ex: LU codes

Nominal data

Numbers with NO numerical meaning

• Ex: LU codes

Ordinal data

Number determining a rank relative to other cells

• Soil drainage (soil is very well drained/poorly drained)

High resolution raster

A raster with smaller cells relative to its size, resulting in a higher total number of cells in a raster. This allows for more precise and accurate analysis.

Low resolution raster

A raster with bigger cells relative to its size, resulting in a lower number of cells in a raster. Analyses with low resolution rasters may result in lower accuracy results.

DEM

Rasters with elevation data of the bare earth.

• Also known as the heart of raster.

DSM

Rasters with elevation data may also depict elevations derived by tree tops building, or other features.

Map algebra

A system used in the raster calculator tool to manipulate raster layers

+ (ADD) in Raster Calculator

Raster equivalent of Geoprocessing Union for vector

* (MULITPLY) in Raster Calculator

Vector equivalent of Geoprocessing Intersect

Raster priority

Using raster calculator to multiply certain layers by factors of 10. You can “union'“ (+) these layers to create a result raster with symbology representing certain criteria met.

• Ex: Conducting an analysis on which locations on a raster are more susceptible to landslides.

Slope percent

The steepness of the terrain expressed in percentage (0-∞%)

Slope in degrees

The steepness of the terrain expressed in degrees (0°-360°)

Aspect

The direction one is facing when going downslope on surface (0°-360°)

Interpolation

Predicting unknown values for a cell in a raster from a limited number of known data points

IDW

Predicting unknown values for a cell in a raster based on its distance relative to known points

NN

• Voronoi (Thiessen) polygons are formed around the center of mass of known points.

• A new polygon is formed about an unknown point

• The areas of existing polygons are weighted based on how much area they contribute to the new polygon.

Spline

A mathematical equivalent of a flexible ruler.

• Inexact: Creates smooth surfaces between points, but may not also represent the known dataset.

Global interpolation

Considers all known values in a dataset to interpolate unknown values

• Ex: Kriging

Local interpolation

Uses nearby data (within a designated search radius) to interpolate unknown values

Exact interpolation

Predicting unknown values while preserving the values of the known dataset

Inexact interpolation

Predicting unknown values without preserving the values of the known dataset

Spatial autocorrolation

Tobler’s First Law of Geography: Features closer to each other are more similar than features far away from each other

Deterministic (non-statistical) interpolation

• Uses mathematical formulas to predict unknown values for points using a set of known points.

• Uses spatial autocorrelation to predict values

• Ex: IDW

Geostatistical interpolation

• Uses the statistical properties of known points to quantify spatial autocorrelation

• Takes into account uncertainty in its predictions

• Ex: Kriging

Sink

A cell with an undefined drainage direction and disrupts the flow of H2O

Pour point

The lowest elevation point of a sink. If the sink were to be filled with H2O, this is where the H2O would pour out.

Why sinks should filled

• To ensure proper delineation of basins and streams

• To identify the boundary of a watershed

Set Null

A tool identifies cells with a known value to be set to “NoData” (null)

Flow direction

Designates a direction of the flow of H2O in a filled DEM

D8

Single directional flow algorithm: A method of calculating flow direction, assigning flow direction to its steepest descent in 1 of 8 directions

D8 directional values

1 = east

2 = south east

4 = south

8 = south west

16 = west

32 = north west

64 = north

128 = north east

D8 is too simple

• Only assigns a cell to flow in one direction, not realistic!

Flow accumulation

Calculating a value with the # of cells that drain into that cell

Upslope area

(The number of cells) x (Grid cell resolution) = the area of land that drains into a particular cell

Grid size resolution

The size of one cell on a raster grid

How flow accumulation can be used in spatial hydrology

Create a stream network (using Raster calculator) to identify flow accumulations above a certain threshold.

CON

Performs a conditional evaluation on each input cells of an input raster

“IF ___, THEN ___.”

How are CON and SetNull used together in spatial hydrology?

To identify cells with a certain value and reclassify them as “NoData”

2 methods of stream order

Strahler and Shreve

Shreve method

• Stream orders are additive downslope.

• When 2 links intersect, their orders are added, regardless if they are of the same order or not.

• The 2nd stream order method created

Strahler method

• Stream orders increases when links of the same order intersect.

• Links with different stream orders will maintain the highest stream order downslope

• The most common method!

• The original stream order method

Which stream order method is more likely to result in more stream orders (1, 2, 3, and so on…)

Shreve. Stream orders of streams that intersect are additive, regardless of if they’re the same stream order or not.

Spatial hydrology tool workflow in ArcGIS Pro

Unfilled DEM → Fill DEM
→Raster calculator → Residual grid

Filled DEM → Set Null
→Flow direction grid
→Flow accumulation grid
→CON to identify stream network
→Set Null: non-stream cells → INPUT STREAM RASTER
→ Stream order (Strahler or Shreve)

FD → Basin to delineate all drainage basins

Basin

Creates a raster delineating all drainage basins

Fill

Fills all sinks in a surface raster

Stream order

Assigns a numeric order to linked within a stream network. Stream orders may increase when links intersect.

Triangulation

A method of precisely locating something using 3 or more satellites

Errors in GPS data

• Blocked signal

• Molecules in our atmosphere messing with radio signals

• Multipath error

• Satellite geometry (too few satellites/poor satellite elevation)

Multipath error

Satellite signals hit things on their path to the object being located → inaccurate location due to longer travel time

Differentiation GPS

Utilizing ground-based reference stations alongside satellites to increase GPS accuracy.

• The more satellites/reference stations in contact, the more accurate the location.

Error

Getting something wrong (Mistakes = inevitable)

• Errors left unchecked → GIS analyses’ results are questionable

Uncertainty

Our lack of knowledge (“I don’t know”)

Accuracy

The degree to which information on a map matches the truth.

Precision

The level of exactness of information on a map

• Repeated samples are near the same value, but may not necessarily reflect reality.

• Ex: Not using the right datum → poor accuracy, but high precision.

Sources of error in spatial data

• Grid cell resolution

• Density of observations (ex: number of weather stations)

• Areal coverage (ex: gaps in DEM coverage)

• Outdated data

Map accuracy standard for maps with a scale greater than 1:20,000 (more zoomed in)

No more than 10% of the points may be off by 1/30th of an inch

Map accuracy standard for maps with a scale less than 1:20,000 (more zoomed out)

No more than 10% of the points may be off by 1/50th of an inch

Small scale

Less detail; more zoomed out

Large scale

More detail; more zoomed in