Spatial Data Management II

Transcription

1 Spatial Data Management II GEOG 300, Lecture 7 Dr. Anthony Jjumba 1 Interested in a Career in Geosciences? Wednesday September 26, :30-4:20 pm Room Please contact: Faran.Ali@unbc.ca Andrea Michaud Student Program Coordinator at Engineers and Geoscientists BC Please join for a discussion about the career benefits of being a professional geoscientist Who is a Geoscientist? Why become a P.Geo? Benefits of a professional designation Membership criteria How to apply? 1

2 Exams Mid-Term: Wed 17 th Oct Final: Wed 28 th Nov Today Data Collection Sampling Scales of measurement Some data management tools 3 Sampling Selecting a representative part of a population for statistical analysis; various designs of sampling can be applied random sampling systematic sampling stratified sampling Within the above designs, one may decide on point line or area method 4 2

3 Sampling Provides knowledge about a whole population i.e. make inference about a population from the sample data Larger sample sizes are more accurate representations of the whole Large samples are costly: time, labour Can be wasteful since we can statistically infer from appropriate samples A sampling strategy with the minimum bias is 5 the most statistically valid Sampling Spatial sample designs: (A) simple random sampling, (B) systematic sampling, (C) stratified random sampling, (D) stratified sampling with random variation in grid spacing, (E) clustered sampling, (F) transect sampling, and (G) contour sampling. Source: Longley, Paul A.; Goodchild, Michael F.; Maguire, David J.; Rhind, David W.. Geographic Information Science and Systems, 4th Edition. Wiley. 6 3

4 Random Sampling Random sampling: each member of the population has an equal chance of being selected Advantages: Can be used with large sample populations Avoids bias Disadvantages: Can disproportionately represent some parts of the population at the expense of others 7 8 4

5 Systematic Sampling Systematic Sampling: Samples are chosen at regular intervals Sample locations are evenly for example every two metres along a transect line systematic sampling implies a regularly spaced grid Advantages: It is more straight-forward than random sampling Provides a good coverage of the study area Disadvantages: It is more biased: not all points have an equal chance of being selected It may lead to over or under representation if there is periodicity in the data (e.g. sampling at the same interval as the location of 9 erosion barriers along a beach. Or a city road grid) 10 5

6 Stratified sampling Stratified sampling: used when the parent population is made up of sub-groups that of interest. Divide the sampling design into strata(classes), and then select a sample from each stratum The strata are defined so that individuals inside each class are similar based on the characteristic believed to influence the phenomena

7 Stratified sampling Advantages: If the proportions of the subgroups are known, the results are representative of the whole population Correlations and comparisons can be made between subgroups Disadvantages: The proportions of the subgroups must be known 13 Air Photos for Stratified Sampling Looking for distinct, uniform areas Crown size (age), harvest history Hardwoods (gray) and softwoods (green) 7

8 Stratified Sampling Generate sample points randomly X points per area, e.g. 1 point every 3 hectares Each point tied to polygon = unique stand Stratified Sampling: Population A E B C D 8

9 Point Sampling Methods 17 Source: J. Chapman, Jr. McGrew. An Introduction to Statistical Problem Solving in Geography Hybrid Point Sampling Methods Source: J. Chapman, Jr. McGrew. An Introduction to Statistical Problem Solving in Geography 18 9

10 Source: 19 Levels or Scales of Measurement Nominal Categorical data e.g. land use type, religious affiliation Ordinal Ranked data, e.g. main, secondary, minor roads Interval: Interval between any two units can be measured on scale. Zero value is assigned arbitrarily e.g. Celsius and Fahrenheit scales (80 o F is not twice as hot as 40 o F) Ratio: interval data with an absolute zero value 20 10

11 Source: J. Chapman, Jr. McGrew. An Introduction to Statistical Problem Solving in Geography 21 GIS Attribute field types: Interval/ Ratio: Integer (long/short) Short: - 32,000 to +32,000 Long: - billions to + billions Real (float/double) Float: precision - field length; scale - number of decimals Double: 15 digits Nominal: Text, (includes code number) Ordinal: Date, Text, (short integer) 11

12 Tools for Data Management Arc Toolbox is clunky Geoprocessing>search for tools Problem: Oversized Data National data > provincial project Provincial data > town project Solutions: Clip, Erase and Intersect 12

13 Clip: Vector Data When you ve got data for too much area Use an existing shapefile or draw a new polygon Recalculate geometry after CLIP Attribute table has not changed lengths, areas etc.. Caution Safety New file (vs. overwriting) Danger New file has old attributes RECALCULATE AREAS NOT an automatic feature 13

14 Raster Clip: Same but different tool Raster clip: define rectangle outline Top left, bottom right (coordinates) Or use a defining polygon e.g park boundary, forest district Erase: Opposite of Clip Eliminate an area from analysis Say you don t care about parks in towns 14

15 Intersect: When you only want the overlapping bit Say you only want parks inside city limits Wildfires within a certain forest type Includes attributes from both layers Problem: post-processed areas Area is stored as an attribute If you clip/erase half a polygon The attribute stays the same The actual area changes Solution: Recalculate areas 15

16 Problem: Incomplete Data Project covers two mapsheets New data has arrived Solutions: Append, Merge Append: When you get new data append -> existing layer Layers may be adjacent or overlap They can be points, lines or polygons but both must be the same feature type They could include adding features to an existing dataset e.g. extra streams, trails to roads. 16

17 Merge: When it comes in two or more files Merge adjacent tiles -> new layer They can be points, lines or polygons but both must be the same feature type Merge Caution: Like a jig-saw, the pieces must fit (spatially) and the (picture) attributes match Ensure you still have the same attributes after merge or append (you may lose them if the parts don t match) 17

18 Example of mismatching attributes: contour lines in feet versus metres (Northern BC NTDB data) Elevation (Feet) Elevation (Metres) In attribute table (old map sheet), add a new field, name it new-elev In the field calculator, new-elev = elevation *.3048 Drop (delete) elevation field Rename new-elev to elevation Merge, Append, Union? What is the difference between merge, append and union? 36 18

19 Problem : Too Much Detail County data on a world map Unnecessary boundaries in homogenous area Thousands of tiny lakes on a provincial map Solutions: Dissolve, Select by Attribute Dissolve: Remove unwanted boundaries Dissolve field above = color If you don t select a dissolve field?? 19

20 Select by Attributes Data>Export data Export selected Problem: Multipart Features Multiple features, one attribute row Multipart Features Changing/symbolizing one affects all parts Alaska, Hawaii, Lower 48: usually multipart You want to show Hawaii differently? Solution: Explode 20

21 Multipart features One attribute record, several features Very helpful? Most annoying thing ever? Explode! -> multipart to single features Problem: Obsolete Data New boundaries + attributes resulting from: Timber harvest Wildfire Prescribed fire Construction Rezoning Solution: Update 21

22 Update: After the buncher (harvester) leaves Overrides old data with new Update Feature Problem: Wrong Units Hectares when you need acres m 2 when you want km 2 Feet when you need meters Solutions: Field Calculator (apply conversion factor) Calculate Geometry (make new measurements) 22

23 Unit Conversion with Field Calc. Keep old measurements, convert units Create a new item for converted units Elevation(feet)=Elevation(meters)* Calculate Geometry New measurements, new units GCS: Geographic Coordinate System PCS: Projected Coordinate System 23

24 Problem: Excess Items Some layers have hundreds of items Many automatically generated, not needed Solution: turn off fields, export to new file Exporting w/o Excess Fields Data>Export Data 24

25 Problem: Wrong Field Name On import, names often truncated Arc won t let you rename the field Solution: Change alias, or make a new field Alias vs New Name Field properties allows alias change, not name New field>field calculator>new field=old field 25

26 Copy/Paste Features Point to point, line to line, poly to poly Plus: poly to line Attributes copied if schema is the same Area column will go to Area column if present If not present, attribute dropped If columns in destination layer blank if not filled Cut/Paste Features Geometry only, no attributes Otherwise the same procedure: 26

27 Delete Features Must be in editing mode Edit features>start editing Select features Attribute table Select tool Select by location Select by attribute Delete key (not backspace) Control-Z works Batch Project When you ve been lazy Ignoring the warning: projection does not match Reproject all your layers at once 27

28 Data Backup At UNBC In the Cloud External hard drives Folder Management When it s just yours: personal preference But do think about it and plan ahead K:/user/Geog300/FinalProject/data1 K:/user/GIS/data1 28

29 Environmental Consulting Data Within-company transfers Between-company transfers Metadata Data about data Coordinate system Chain-of-custody 29