Research Preview — Code snippets need testing against actual implementation.

Build a Local Currency

Create a local currency that can only be traded by people physically in a specific region — enabling hyperlocal economies.

About location verification: This guide uses GPS coordinates as input. GPS is spoofable. We’re working on Location Proof plugins that will replace getCurrentLocation for stronger verification — these are still in development.

Concept

Imagine a “Berlin Token” (BLN) that:

Can only be minted by people present in Berlin
Can only be transferred to people currently in Berlin
Creates a true local currency for the city

For more on the economic theory behind spatially-restricted currencies, see Spatial Demurrage.

Step 1: Define the Region

import { AstralSDK } from '@decentralized-geo/astral-sdk';

const astral = new AstralSDK({ chainId: 84532, signer: wallet });

// Berlin city boundary (simplified)
const berlinBoundary = {
  type: 'Polygon',
  coordinates: [[[
    [13.088, 52.338],
    [13.761, 52.338],
    [13.761, 52.675],
    [13.088, 52.675],
    [13.088, 52.338]
  ]]]
};

// Create region attestation
const region = await astral.location.onchain.create({
  location: berlinBoundary,
  memo: "Berlin city boundary for local currency"
});

console.log('Region UID:', region.uid);

Step 2: The Token Contract

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

import "@eas/contracts/resolver/SchemaResolver.sol";
import "@openzeppelin/contracts/token/ERC20/ERC20.sol";
import "@openzeppelin/contracts/access/Ownable.sol";

contract GeofencedToken is SchemaResolver, ERC20, Ownable {
    address public astralSigner;
    bytes32 public regionUID;

    // Track pending operations
    mapping(bytes32 => PendingMint) public pendingMints;
    mapping(bytes32 => PendingTransfer) public pendingTransfers;

    struct PendingMint {
        address recipient;
        uint256 amount;
        bool exists;
    }

    struct PendingTransfer {
        address from;
        address to;
        uint256 amount;
        bool exists;
    }

    event MintRequested(bytes32 indexed requestId, address recipient, uint256 amount);
    event TransferRequested(bytes32 indexed requestId, address from, address to, uint256 amount);

    constructor(
        IEAS eas,
        address _astralSigner,
        bytes32 _regionUID
    )
        SchemaResolver(eas)
        ERC20("Berlin Token", "BLN")
        Ownable(msg.sender)
    {
        astralSigner = _astralSigner;
        regionUID = _regionUID;
    }

    // Request a mint - user must then prove they're in the region
    function requestMint(uint256 amount) external returns (bytes32 requestId) {
        requestId = keccak256(abi.encode(msg.sender, amount, block.timestamp));
        pendingMints[requestId] = PendingMint({
            recipient: msg.sender,
            amount: amount,
            exists: true
        });
        emit MintRequested(requestId, msg.sender, amount);
    }

    // Request a transfer - recipient must prove they're in the region
    function requestTransfer(address to, uint256 amount) external returns (bytes32 requestId) {
        require(balanceOf(msg.sender) >= amount, "Insufficient balance");
        requestId = keccak256(abi.encode(msg.sender, to, amount, block.timestamp));
        pendingTransfers[requestId] = PendingTransfer({
            from: msg.sender,
            to: to,
            amount: amount,
            exists: true
        });

        // Lock tokens during pending period
        _transfer(msg.sender, address(this), amount);

        emit TransferRequested(requestId, msg.sender, to, amount);
    }

    function onAttest(
        Attestation calldata attestation,
        uint256 /*value*/
    ) internal override returns (bool) {
        require(attestation.attester == astralSigner, "Not from Astral");

        // Decode policy attestation
        (
            bool inRegion,
            bytes32[] memory inputRefs,
            uint64 timestamp,
            string memory operation
        ) = abi.decode(
            attestation.data,
            (bool, bytes32[], uint64, string)
        );

        require(inRegion, "Not in region");
        require(inputRefs[0] == regionUID, "Wrong region");
        require(timestamp > block.timestamp - 1 hours, "Too old");

        // Extract request ID from recipient field (encoded)
        bytes32 requestId = bytes32(uint256(uint160(attestation.recipient)));

        // Check if this is a mint or transfer
        if (pendingMints[requestId].exists) {
            _executeMint(requestId);
        } else if (pendingTransfers[requestId].exists) {
            _executeTransfer(requestId);
        } else {
            revert("No pending operation");
        }

        return true;
    }

    function _executeMint(bytes32 requestId) internal {
        PendingMint memory pending = pendingMints[requestId];
        delete pendingMints[requestId];
        _mint(pending.recipient, pending.amount);
    }

    function _executeTransfer(bytes32 requestId) internal {
        PendingTransfer memory pending = pendingTransfers[requestId];
        delete pendingTransfers[requestId];
        _transfer(address(this), pending.to, pending.amount);
    }

    // Cancel pending operations
    function cancelMint(bytes32 requestId) external {
        require(pendingMints[requestId].recipient == msg.sender, "Not your request");
        delete pendingMints[requestId];
    }

    function cancelTransfer(bytes32 requestId) external {
        PendingTransfer memory pending = pendingTransfers[requestId];
        require(pending.from == msg.sender, "Not your request");
        delete pendingTransfers[requestId];
        _transfer(address(this), pending.from, pending.amount);
    }

    function onRevoke(Attestation calldata, uint256) internal pure override returns (bool) {
        return false;
    }
}

Step 3: SDK Integration

async function mintLocalToken(amount: bigint, wallet: Signer) {
  const astral = new AstralSDK({ chainId: 84532, signer: wallet });
  const tokenContract = new Contract(TOKEN_ADDRESS, TOKEN_ABI, wallet);

  // 1. Request mint
  const tx1 = await tokenContract.requestMint(amount);
  const receipt = await tx1.wait();
  const requestId = receipt.logs[0].args.requestId;

  // 2. Get user's current location
  const coords = await getCurrentLocation();

  // 3. Create location attestation
  const userLocation = await astral.location.onchain.create({
    location: { type: 'Point', coordinates: coords }
  });

  // 4. Prove they're in the region
  const proof = await astral.compute.contains(
    REGION_UID,
    userLocation.uid,
    {
      schema: SCHEMA_UID,
      recipient: requestId  // Pass request ID as recipient
    }
  );

  if (!proof.result) {
    await tokenContract.cancelMint(requestId);
    throw new Error('Not in the required region');
  }

  // 5. Submit proof → triggers mint
  const tx2 = await astral.compute.submit(proof.delegatedAttestation);
  await tx2.wait();

  return { transactionHash: tx2.hash };
}

Alternative: Simpler Transfer Gate

For a simpler implementation, gate transfers directly:

contract SimpleGeofencedToken is ERC20 {
    IEAS public eas;
    address public astralSigner;
    bytes32 public regionUID;
    bytes32 public schemaUID;

    function transferWithProof(
        address to,
        uint256 amount,
        bytes32 attestationUID
    ) external {
        // Fetch attestation from EAS
        Attestation memory att = eas.getAttestation(attestationUID);

        // Verify
        require(att.attester == astralSigner, "Invalid attester");
        require(att.recipient == to, "Wrong recipient");

        (bool inRegion, bytes32[] memory inputs, , ) = abi.decode(
            att.data,
            (bool, bytes32[], uint64, string)
        );

        require(inRegion, "Recipient not in region");
        require(inputs[0] == regionUID, "Wrong region");

        // Execute transfer
        _transfer(msg.sender, to, amount);
    }
}

Use Cases

Neighborhood Currency

Local money that stays in the community

Tourism Credits

Tokens only spendable within city limits

Regional Stablecoins

Pegged currencies for specific regions

Event Tokens

Festival/conference currency

Next: Delivery Verification

Build an escrow with location verification

​Build a Local Currency

​Concept

​Step 1: Define the Region

​Step 2: The Token Contract

​Step 3: SDK Integration

​Alternative: Simpler Transfer Gate

​Use Cases

Neighborhood Currency

Tourism Credits

Regional Stablecoins

Event Tokens

Next: Delivery Verification

Build a Local Currency

Concept

Step 1: Define the Region

Step 2: The Token Contract

Step 3: SDK Integration

Alternative: Simpler Transfer Gate

Use Cases