From Delphi to React: Modernizing Desktop GIS and Mapping Applications

The "Big Bang" rewrite is a suicide mission for Enterprise GIS. If you are still trying to move from Delphi to a modern React-based mapping stack by manually auditing 20-year-old Object Pascal files, you aren't modernizing—you're performing digital archaeology at $200 an hour.

The global technical debt burden has reached $3.6 trillion, and nowhere is this more visible than in the specialized world of desktop Geographic Information Systems (GIS). These systems, often built in Delphi during the late 90s and early 2000s, house mission-critical spatial logic, coordinate transformation algorithms, and proprietary rendering pipelines that no one currently on your payroll fully understands.

When 70% of legacy rewrites fail or exceed their timelines, the traditional approach of "read the code, write the requirements, build the new app" is no longer a viable strategy. It is a fiscal liability.

TL;DR: Modernizing legacy Delphi GIS applications requires moving away from manual code audits toward Visual Reverse Engineering, reducing migration timelines from years to weeks by using video as the source of truth for component extraction.

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The Delphi Trap: Why GIS Modernization Stalls#

Delphi was the gold standard for desktop GIS because of its rapid UI development and low-level memory management. However, those same features have created a "black box" effect. In many Financial Services and Government organizations, the original architects of these systems have retired, leaving behind 67% of legacy systems with zero documentation.

The challenge of moving from Delphi to React isn't just a syntax change; it’s a paradigm shift from stateful desktop environments to stateless web architectures. Traditional modernization attempts stall because:

1. •Spatial Logic Entanglement: Business logic for calculating buffers, intersections, or projections is often tightly coupled with the VCL (Visual Component Library) UI code.
2. •The Documentation Gap: Manual documentation takes an average of 40 hours per screen. For a GIS suite with 100+ specialized toolbars and dialogs, you’re looking at two years of "discovery" before a single line of React is written.
3. •Data Integrity Risks: GIS systems handle complex coordinate systems (WGS84, NAD83). A minor error in translating the legacy Delphi math to TypeScript can result in catastrophic mapping errors for telecom or utility infrastructure.

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Stop Archaeology, Start Extraction#

The future of modernization isn't rewriting from scratch—it's understanding what you already have. This is where Replay changes the math. Instead of hiring a team of consultants to spend six months reading

and files, Replay uses Visual Reverse Engineering.

By recording a real user workflow—such as a technician performing a spatial query or a land surveyor updating a parcel boundary—Replay captures the "source of truth." It doesn't care how messy the legacy Delphi code is; it focuses on the inputs, the outputs, and the UI state.

💰 ROI Insight: Manual screen documentation costs roughly $6,000 per screen (40 hours @ $150/hr). Replay reduces this to 4 hours, saving $5,400 per screen. For a 50-screen enterprise app, that is $270,000 in immediate savings.

From Black Box to Documented Codebase#

When moving from Delphi to React, the most critical asset is the "Flow." In a GIS context, a flow might be: Select Layer -> Filter by Attribute -> Export to Shapefile.

Replay records these interactions and automatically generates:

• •API Contracts: What data did the Delphi app request from the spatial database?
• •React Components: Clean, documented UI components that mirror the legacy functionality but use modern design systems.
• •E2E Tests: Automated tests that ensure the new React app behaves exactly like the old Delphi app.

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The Technical Blueprint: Transitioning the Logic#

Let’s look at what this looks like in practice. Below is a conceptual example of how a legacy Delphi spatial calculation is extracted and transformed into a modern, type-safe React component using Replay’s automation suite.

Legacy Delphi Logic (The "Black Box")#

pascal
// Legacy Delphi procedure for calculating distance between points
// Hidden in a 5,000 line Unit1.pas file
procedure TMapForm.CalculateDistance(lat1, lon1, lat2, lon2: Double);
var
  dLat, dLon, a, c: Double;
begin
  // Complex Haversine math often undocumented
  dLat := (lat2 - lat1) * PI / 180;
  dLon := (lon2 - lon1) * PI / 180;
  a := Sin(dLat/2) * Sin(dLat/2) +
       Cos(lat1 * PI / 180) * Cos(lat2 * PI / 180) *
       Sin(dLon/2) * Sin(dLon/2);
  c := 2 * ArcTan2(Sqrt(a), Sqrt(1-a));
  ResultValue := 6371 * c; // Earth radius in km
  DistanceLabel.Caption := FloatToStr(ResultValue);
end;

Extracted React Component (The Replay Output)#

Replay identifies the UI elements and the underlying logic to generate a clean, modular React component.

typescript
import React, { useState, useEffect } from 'react';
import { calculateHaversine } from './spatial-utils';

interface DistanceCalculatorProps {
  point1: [number, number];
  point2: [number, number];
  onCalculationComplete?: (distance: number) => void;
}

/**
 * Generated by Replay Visual Reverse Engineering
 * Source: TMapForm.CalculateDistance (Legacy Delphi)
 */
export const DistanceCalculator: React.FC<DistanceCalculatorProps> = ({ 
  point1, 
  point2,
  onCalculationComplete 
}) => {
  const [distance, setDistance] = useState<number | null>(null);

  useEffect(() => {
    const result = calculateHaversine(point1, point2);
    setDistance(result);
    if (onCalculationComplete) onCalculationComplete(result);
  }, [point1, point2]);

  return (
    <div className="p-4 border rounded shadow-sm bg-white">
      <h3 className="text-sm font-semibold text-gray-700">Spatial Calculation</h3>
      <div className="mt-2 text-2xl font-bold text-blue-600">
        {distance ? `${distance.toFixed(2)} km` : 'Calculating...'}
      </div>
    </div>
  );
};

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3 Steps to Modernize Your GIS Stack#

If you are a CTO or VP of Engineering, you don't have 18 months to wait for a "Big Bang" release. You need incremental value. Here is the framework for using Replay to move from Delphi to React.

Step 1: Visual Audit and Recording#

Instead of a code audit, perform a "Workflow Audit." Use Replay to record expert users performing the most complex tasks in the Delphi application. This captures the edge cases that are never found in the requirements documents.

Step 2: Component Extraction and Design System Mapping#

Replay’s Library feature identifies recurring UI patterns in the legacy GIS—toolbars, layer pickers, coordinate displays—and maps them to your modern React design system. This ensures that the 70% time savings are realized by not building the same button 50 times.

Step 3: API Contract Generation#

GIS systems are data-heavy. Replay observes the network and database calls made during the recording to generate an API contract. This allows your backend team to build the necessary microservices in parallel with the frontend development, knowing exactly what data structures are required.

⚠️ Warning: Do not attempt to "improve" the business logic during the extraction phase. The primary goal is parity. Once you have moved the logic from a Delphi black box to a documented React codebase, you can optimize.

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Addressing the Regulated Environment#

For our clients in Healthcare, Financial Services, and Government, security isn't an afterthought—it's the barrier to entry. Legacy Delphi systems often survive because they are "air-gapped" or run on-premise.

Replay is built for these environments. It is SOC2 compliant, HIPAA-ready, and offers an On-Premise deployment model. Your proprietary spatial data and legacy source code never need to leave your secure network. You get the power of AI-driven automation without the risk of data exposure.

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Challenging the Conventional Wisdom#

The industry will tell you that you need to hire a specialized Delphi-to-Web migration firm. They will quote you $2M and a 24-month timeline. They will spend the first six months "learning your business."

They are wrong.

Your business logic is already documented—it’s just documented in the workflows of your users. By using video as the source of truth, Replay bypasses the "discovery" phase entirely. We move the conversation from "What does this code do?" to "This is what the user does; now let's generate the code to support it."

💡 Pro Tip: Focus your first 2-week sprint on the "Map Viewer" component. It’s the highest value and highest complexity. Using Replay to extract the layer management logic will prove the ROI to stakeholders faster than any slide deck.

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Frequently Asked Questions#

How long does legacy extraction take?#

For a standard enterprise screen, manual reverse engineering takes 40 hours. With Replay, we reduce that to 4 hours. A full GIS module that would typically take 6 months can be ready for development in 2-3 weeks.

What about business logic preservation?#

Replay doesn't just look at the UI; it monitors the state changes and data flows. By capturing the inputs and outputs of the Delphi procedures, it generates React logic that preserves the original business intent, even if the underlying Pascal code is spaghetti.

Does this work with specialized GIS components?#

Yes. Whether your legacy app uses Esri MapObjects, MapInfo components, or custom VCL drawing routines, Replay captures the rendered output and the interaction patterns, allowing you to map them to modern libraries like Mapbox GL JS or OpenLayers.

We have no documentation. Is that a problem?#

No. In fact, that's where Replay excels. 67% of our clients have little to no documentation. Replay becomes your documentation by creating a visual and technical map of the legacy system through user recordings.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.