Geo crash course

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smartpuffin.com | @ice_lenor | #geosmartpuffin.com | @ice_lenor | #geo
Geo Crash Course
Elena Nikolaeva

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Who am I?
● 10 years in Geo
● Started in 2GIS
● Built a distributed scalable
GIS-syste...

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Purpose:
make you familiar
with Geo

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What is an integer?
● A whole number
● 4 bytes (nowadays, mostly)
● Unsigned or signed...

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Common wisdom
● One doesn't store money in double
● When working with strings, mind th...

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Geo data: it's a
data type!

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Geometry types

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Point
Point(25 10)

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MultiPoint
1
2
point1
point2
MultiPoint( (25 10), (32 9) )

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LineString
1
2
LineString(5 10, 22 15)
● Consists of ordered points

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MultiLineString
1
2
3
4
line1 line2
MultiLineString( (10 10, 20 20), (30 15, 40 12) )

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Polygon
1
3
2
4
Polygon((0 0, 2 5, 4 0, 0 0))
● Consists of ordered points; certain di...

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Polygon with holes
6
7
8
9
1
2
3
4
5
Polygon((0 0, 8 0, 8 5, 0 5, 0 0), (3 2, 4 5, 6 2...

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1
2
3
4
5
MultiPolygon
10
6
7 8
9
11
12 13
14
15
16
17 18
poly1
poly2
poly3

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GeometryCollection
GeometryCollection(
Polygon((...)),
LineString(...),
Point(...),
Po...

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● Name: Amsterdam
● Type: city
● Population: 821,752
● Country: The Netherlands
GeoObj...

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Coordinates

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Who thinks that
Earth is flat?

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SHOCK!

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(x, y)
Point (0 10)
LineString (0 0, 10 0)
Flat coordinates

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(lat, long)
Point (52.373160 4.891999)
52.3680° N, 4.9036° E
Geography coordinates

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Flat coordinates
● Computer games
● Building plans
● Large scale maps of small areas
○...

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Flat coordinates
(0, 0) 5 10 15 20 25
5
10
15
Ficus is at
(23, 8)
x
y

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Geographical coordinates
● Map of the whole planet
● Map of a large area of the planet...

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Small scale: when everything is small

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Large scale: when everything is LARGE

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Latitude
0°
90° North
-90° South
North Pole
South Pole
30th parallel North
30° N
30°
4...

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Longitude
0°
-180°
West
180°
East
40th meridian east
40° E
40°20th meridian west
40° W...

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Datums and
Coordinate Systems
How round is Earth, e...

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Geoid
Earth's shape is a

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Spheroid
But we can simplify it to a

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WGS84
The latest revision is WGS 84 (also known as WGS
1984, EPSG:4326). The WGS 84 da...

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WGS84
radius
R = 6378 km
flattening
F = 1 / 298.25
● Earth is an ellipsoid
○ flattened...

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Datum describes Earth's
general shape
(Sphere? Ellipsoid? Spheroid?)

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Coordinate system describes
where coordinates start
where they end
and what’s the unit

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You must specify
the Coordinate System
to make your math correct!

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Formula for distance: flat
Point(0 0) to Point(1 1) = 1.414 (units)
● We all learned i...

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Formula for distance: haversine
● Uses radius
● Doesn’t use flattening
● => Calculates...

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Formula for distance: Vincenty
● Uses radius
● And flattening
● => Calculates on WGS84...

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SRID: Spatial Reference Id
● EPSG - European Petroleum Survey
Group
○ A registry of da...

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Why is a SRID like a toothbrush?
Because everyone has got one.
● British National Grid...

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Who still thinks
that
Earth is flat?

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Projections

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"How do we draw
a 3D-Earth
on paper?"
- Gerardus Mercator

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Projection
● A means to display a N-dimensional object
in a (N-1)-dimensional space
● ...

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We take a datum
and flatten it!

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Projections
● A sphere can't be flattened without problems
○ You have to tug and stret...

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Specific way of flattening,
shrinking, stretching, fitting
is called
a projection

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Mercator Projection
● Preserves angles and shapes of
small objects
● => Good for navig...

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Orthographic
● "Like I'm looking at it from
space!"
● Directions from the central
poin...

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Plate Carrée
● longitude => y
● latitude => x
● Preserves angles
● Loses everything
el...

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Equal-area (i.e., Mollweide, Lambert, etc)
● Preserves
relative areas
● Distorts angle...

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Winkel Tripel: A Compromise?
● Tries to preserve all at once
● Minimises distortion of...

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Universal Transverse Mercator (UTM)
● Family of 60 projections ("zones")
○ Slice Earth...

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There is no single
"best projection"

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Each projection
is good
for a specific purpose

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Web Mercator: A Cursed Projection :)
● "Invented" by Google
○ They mix sphere and elli...

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OGC Standard

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OGC Standard
● Open Geospatial Consortium: 500+ companies
● De-facto defines the geo-i...

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What can I do with my geodata?
Geo Math:
Spatial Op...

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Distance
Point(0 0)
Point(4, 2)
Distance = 222.4 km
(calculated on WGS84)
Distance = 4...

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Inside

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Touches

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Overlaps

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Topological operations: Union
∪
poly1
poly2
poly1

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Topological operations: Intersection
∩
poly1
poly2

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Topological operations: Difference
poly1
poly2

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Bounding box
Geometry
Bounding box:
smallest enclosing
rectangle

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Buffer(5)
Buffer
5
5

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● Coordinates must be within SRID's limits
○ I.e., 95° North latitude is a no-no
● Lin...

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1
5
2
IsValid, MakeValid: Self-Intersection
3
4
● 8-shape polygon
● "Butterfly" geomet...

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Storing spatial data

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Well-known geometry representation
● Defined in OGC standard
● Well-known text
○ "Poin...

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Spatial databases
● OGC-compliant
● Store and index spatial data
○ Fast and correct qu...

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Spatial databases
● SQL Server
● Postgres: PostGIS extension
● Oracle Spatial and Grap...

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Spatial Elasticsearch?
● Supports only geographical coordinates on
WGS84
● Index: GeoH...

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Spatial indexes
● Quickly answer questions:
○ "What is close to this place"
○ "What is...

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Spatial index: Quadtree
● Divides space into 4
squares recursively
● Adding new object...

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Spatial index: R-tree
● Divides space into
rectangles
● Efficient for reading
● Adding...

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Spatial index: Z-order Curve
● Efficient for reading
● And for writing
● Harder to com...

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File formats
● Download your geodata into a file (like .csv)
● Then upload it somewher...

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Libraries
● OGC defines the API
● Can load a geometry from spatial db
● Can operate on...

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GIS-systems
Geographic information
systems: apps for creating
and editing geodata.
● A...

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GIS-systems
● Users: geo-specialists (cartographers)
● Create layers (types of objects...

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Common Problems in Geo

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new Point (lat, lon)
OR
new Point (lon, lat)
?
● Library-dependent
● Always check!

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Units: degrees
● Units of latitude and longitude: degrees
● You CAN'T measure distance...

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Units: degrees
● Two points on the equator:
● Point 1: latitude 0, longitude 0.
● Poin...

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Units: degrees
● Two points on Amsterdam's latitude (52° N):
● Point 1: latitude 52, l...

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The closer to the pole,
the less km's fit
in one degree longitude

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Degrees latitude
are always the same
in km's

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180th Meridian

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180th Meridian: Always Check With Fiji!
-16.760, -179.658
-16.712, 179.713
180th Merid...

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180th Meridian

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180th Meridian
● Fiji
● New Zealand
● Russia
● Kiribati
● United States

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Poles
● One latitude: 90° or -90°
● All longitudes meet there
=> It's all exactly the ...

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Poles
● Some projections don't support poles
● I.e., Mercator on WGS84:
○ "World betwe...

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Spatial operations around problem areas
● Math close to 180th meridian must work
corre...

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Thank you
Your questions, please!

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