DB: Geolocation Coordinates

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来自 Johns Hopkins University 的课程

毕业项目：旅游照片 Web 应用

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Johns Hopkins University

毕业项目：旅游照片 Web 应用

18 个评分

课程 6（共 6 门，Specialization Ruby on Rails Web Development）

In this Capstone project for the Photo Tourist you will implement a Ruby on Rails web application that makes use of both a relational and NoSQL database for the backend and expose the data through services to the Internet using Web services and a responsive user interface operating in a browser from a desktop and mobile device. You will have a chance to revisit and apply what you have learned in our previous courses to build and deploy a fully functional web application to the cloud accessible to your co-workers, future employers, friends, and family. In developing the Photo Tourist web application, you will get to work with different data types and data access scenarios (e.g., fielded data display and update, image upload/download, text search, access controlled information) to provide your users the ability to show off their photos and information from trips they have taken and to seek out photos and information from trips taken by others. Using the application you develop, your users will be able to • Create an account • Upload and download photos to the site and make them accessible to others • Provide descriptions of trips and photos that others can read • Organize photos by location and trip, • Find photos based on location • Find photos based on text searches of descriptions • Locate the place where the photo was taken on a map

从本节课中

Geolocation

In this module you will add geolocation capabilities to the end-to-end application. This will allow your users to assign a geolocation position to images as well as identify the current origin and distance limits for future API queries. You will implement the model and API queries for subjects within this module and complete the UI components within the next module. At the data tier you will add geolocation coordinates to the Image model, make those geolocation coordinates available to ThingImage though the 1:1 relationship between them, implement geolocation queries using the Geokit Rails gem that determine all items within a distance limit from an origin and supply their distance from origin. These queries will leverage custom SQL selects, joins (inner, right outer, and left outer join) and will be encapsulated within ActiveModel query scopes. At the API tier you will implement API service endpoints to geocode addresses and reverse geocode positions into a resolved location using the Geokit Rails gem and the Google Maps Geocode service, implement a persistence cache of geocode results using MongoDB to help stay within free service limits and speed up lookups, enable an API service endpoint to query for subjects (ThingImages) of a certain type, within a distance limit, and dynamically annotate them with the distance from the supplied origin, implement HTTP caching headers for both geocode and subject results. At the UI tier you will implement a geocoder service to lookup a location by (partial) address or position from the API implemented on the Rails server, add geocoding to the Image editor to allow a user to specify the Image's position using an address, build a current origin component to express and retain the center origin and distance limits for future subject queries.

Module 6 Introduction3:05

DB: Geolocation Coordinates4:51

DB: Image Position and Value Objects14:01

DB: Geolocation Queries with Geokit Rails14:19

DB: Associated Object Geolocation Queries13:08

DB: ThingImage Geolocation Query Scopes9:46

DB: PostalAddress and Location Value Objects7:34

与讲师见面

Jim Stafford
Adjunct Professor, Graduate Computer Science
Computer Science, Whiting School of Engineering

[MUSIC]

In this lecture, we will discuss geolocation coordinates, longitude and

latitude, and adding geolocation coordinates to our image object so

we can begin to perform functions based on where things are located.

The geolocation coordinate system is based upon two primary attributes,

longitude and latitude.

Longitude is a value based upon an angle,

in degrees, from Greenwich, England, either east or west.

If you head east toward Asia, you're heading in the positive direction.

If you head west towards North America, you're heading in the minus direction.

For example, if you head towards Baltimore,

Maryland, you're heading to -76 degrees.

Latitude, on the other hand, goes from the equator towards the poles.

If you go towards the south pole, you're heading in the negative direction,

and if you head towards the north pole, you're heading in the positive direction.

For example, to head to Baltimore,

Maryland, you'd be heading in a positive 39 degrees, and

this is where Baltimore Inner Harbor is, based upon the overall map.

We will refer to this latitude and longitude as a position.

Now there seems to be multiple ways in which we can represent these coordinates.

We could represent them as arrays where we put latitude and

then longitude, or longitude and then latitude.

It's a little ambiguous when you take a look at it.

We could implement it as hashes.

And this is a lot more explicit,

where each value is keyed on whether it is latitude and longitude.

I like this.

And then there's the GeoJSON standard for

type point which lists coordinates as arrays.

To me, it's a little ambiguous.

And they list their example on their website as longitude and latitude.

So I'm going to take my key from the GeoJSON site and

say that, if ever I'm being ambiguous, like an array, like parameters,

I'm always going to try to speak about longitude first, and then latitude.

Just to enforce some consistency, because outside of these data types, we could also

have parameters into functions where one could take a latitude and longitude,

or longitude and latitude.

And as I said, for consistency, if I ever do these techniques,

I'm going to take longitude ahead of latitude and then, of course,

also look to be totally unambiguous and accept some kind of type that

represents explicitly which value is latitude and longitude.

That is a much safer way to pass parameters.

So in this overall module, and within this lesson, we are going to implement a few

value objects that represent locations on the Earth.

There are several properties we could have included.

I've chosen three here that are available from our geolocation service.

They are formatted_address, position, and address.

I'll be first worrying about position in this lecture.

Address is a postal address, a simplistic one, street address, city,

state code, zip, and country code.

Position is a point, a lat and lng.

So, if you put them all together, they provide a nice, textual,

descriptive postal address to put on a display.

It provides coordinates to where we can navigate specifically to that location,

and provides fielded information in which,

basically breaks up the formatted_address into pieces.

We're going to start integrating location information by putting position

into images, inserting latitude and longitude.

And so, over the course of this lecture, we're going to be focusing on adding

geolocation properties to our image by adding individual properties,

as well as to express a position implemented by a point value object.

And then, the rest of the module, we'll start worrying about using that

information in order to calculate distances.

All right, let's get started expressing position within our image.

In summary, we introduced geographic coordinates, longitude and

latitude, as a means to identify positions on the Earth.

And identified a location representation that we are going to use throughout this

module and the rest of the application, dealing with formatted address,

position, and a postal address that'll identify the location of things.

And then we will soon represent position, longitude and

latitude, within images, using the point class.

And this will prepare us for use with queries and

calculations that can leverage position information.

What's next?

Image position and value objects.

Let's get down to the implementation details of representing longitude and

latitude within image,

wrapped by a point value object representing a position concept.

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