# HALOWERK Eastern Land – Bulgaria Mean Surface Slope

> HALOWERK Eastern Land – Bulgaria Mean Surface Slope is a paid API for AI agents from ostland.halowerk.com, paid per call via x402, $0.003/call, status unknown (last checked 2026-09-13).

Returns the mean first-order surface gradient (slope) across the analysis grid for a given WGS84 coordinate within Bulgaria.

## Facts

- Endpoint: POST https://ostland.halowerk.com/v1/bg-mean-slope
- Price: $0.003/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-13
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/halowerk-eastern-land-bulgaria-mean-surface-slope-d5573d17
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_MypGbI2toIYkHq3WNvauE

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability halowerk-eastern-land-bulgaria-mean-surface-slope-d5573d17 -d '<json body>'
```

Example prompt: What's the mean surface slope at latitude 42.15, longitude 24.75 in Bulgaria? I need the average first-order terrain gradient for that point.

## When to prefer this

Use this endpoint when you need the mean first-order terrain slope for a specific point in Bulgaria and want a validated, boundary-checked result. Prefer this over generic elevation APIs when you specifically need slope (gradient) rather than elevation, and when the target location is within Bulgaria. Use the max-slope sibling endpoint if you need peak adjacent-cell gradient rather than the mean.

## Known failure modes

- Coordinate outside Bulgaria boundary — request rejected with validation error before payment
- Coordinate within European coverage range but outside Bulgaria polygon — rejected by Natural Earth boundary check
- Latitude or longitude out of published European coverage range (lat 34–72.5, lon -31.5–40.5) — schema validation failure
- Missing required latitude or longitude fields — request rejected with input error
- No data available for remote or sparse grid cells — may return null or error

## How this service works

Mean first-order surface gradient across the analysis grid. The coordinate is validated against the Natural Earth boundary for Bulgaria; results outside it are rejected before payment.

## Output

A numeric value representing the mean first-order surface gradient (slope) across the analysis grid cell containing the specified coordinate. Coordinates outside the Bulgaria boundary are rejected before any payment is processed.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "properties": {
  "latitude": {
   "type": "number",
   "maximum": 72.5,
   "minimum": 34,
   "description": "WGS84 latitude inside the published European coverage."
  },
  "longitude": {
   "type": "number",
   "maximum": 40.5,
   "minimum": -31.5,
   "description": "WGS84 longitude inside the published European coverage."
  }
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/halowerk-eastern-land-bulgaria-mean-surface-slope-d5573d17/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from ostland.halowerk.com](https://www.zero.xyz/host/ostland.halowerk.com/llms.txt)