geodatadownloader.com (GDD)

This repo contains all of the code for https://geodatadownloader.com

What is geodatadownloader?

GDD is client-side browser application that will download all the data contained in an ArcGIS feature layer onto your computer. It is not limited by max query size, and can download any size dataset (yes that includes those huge parcel layers from your local county). You can choose a custom extent for your download, and pick the output columns you want to use.

GDD runs entirely in your browser and stores nothing besides what your browser caches locally. There is no backend to the application, besides the CDN used to serve up the html/javascript. In order to draw an extent the map uses ESRI's javascript library (and therefore ESRI's servers to serve up the data for the map). Conversion to from arcgis json to geojson is done browser side as well.

What formats does this support?

• GeoJSON
• CSV
• SHP (ESRI Shapefile)
• GPKG
• KML
• GPX
• PGDUMP
• DXF
• SQLite

How does geodatadownloader download all of the data for a layer?

It executes a query on the arcgis feature service that says where: 1=1. Or, in other words, return everything. When it executes this query, it specifies the parameter returnOnlyObjectIds. This returns all objectIds in a list. Then, GDD paginates those objectIds into chunks of 500. It then constructs a where: OBJECTID IN (...objectIds...) which returns those 500 features. It then proceeds to do this until every chunk has been requested and written into the downloaded dataset.

You may be asking yourself "Why even ask for the objectIds and instead just grab all the features in that original where: 1=1?". If GDD could, it would. ArcGIS REST services are typically are limited by a specific number of features they can return (usually around 1000, but it depends). Some endpoints are "paginated", meaning that you can fetch features one page at a time. Not all services support this, though. So instead, we use the objectIds method because it works a lot more consistently.

Developer Setup

Prerequisites

• Node.js: Version specified in .nvmrc (v20.19.5)
  • We recommend using nvm to manage Node.js versions
• npm: Comes with Node.js

Installation

1. Clone the repository:

   git clone https://github.com/mchaynes/geodatadownloader.git
   cd geodatadownloader

2. Install Node.js (if using nvm):

   nvm install
   nvm use

3. Install dependencies:

   npm install

Running the Development Server

Start the development server on port 3000:

npm start

The application will be available at http://localhost:3000

Building for Production

Build the application for production:

npm run build

The built files will be in the dist/ directory.

Testing

Unit Tests

Run Jest unit tests:

npm test

End-to-End Tests

Run Playwright E2E tests:

npm run test:e2e

For interactive E2E testing:

npm run test:e2e:ui

For headed browser mode:

npm run test:e2e:headed

Linting

Run ESLint to check code quality:

npm run lint

Project Structure

• app/ - Main application code
• src/ - Legacy test setup files
• public/ - Static assets
• tests/ - Test files

Technology Stack

• Framework: React 18 with TypeScript
• Build Tool: Vite
• Styling: Tailwind CSS, Material-UI, Flowbite
• Maps: ArcGIS API for JavaScript
• Data Processing: GDAL WebAssembly (gdal3.js)
• Testing: Jest (unit), Playwright (E2E)

Contributing

Please ensure your code passes linting and all tests before submitting a pull request.